### Description
This PR does several things:
- it cleans up some structural issues with the engineering documentation:
- it harmonizes the table of contents structure across different files
- adds a link to the bigquery documentation
- moves some files to a new `deep-dives` subfolder
- puts a title at the top of each page to avoid github assuming their title is "table of contents"
- it pre-fills the glossary with a long list of words that could use an entry (all empty for now)
- it adds the only remaining relevant server file from [hasura-internal's wiki](https://github.com/hasura/graphql-engine-internal/wiki): the old "multiple backends architecture" file
### Discussion
A few things worth discussing in the scope of this PR:
- is it worth migrating old documentation such as the multiple backends architecture, that document a decision process rather instead of being up-to-date reflections of the code? are we planning to delete hasura-internal?
- should we focus instead on _new_ documentation, aimed to be kept up to date?
- are there other old documents we want to move in here, or is that it?
- is this glossary structure ok, or would a purely alphabetical structure make sense?
- does it make sense to have the glossary only in the engineering section? more generally, _what's our broader plan for documentation_?
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4537
GitOrigin-RevId: c2b674657b19af7a75f66a2a304c80c30f5b0afb
### Description
When generalizing the code, back in late 2020, we over-eagerly generalized parts of the code that are specific to RQL's DML. This was in part due to the fact that, at the time, the DML types were all mixed alongside other types in `RQL.Types`. As a result, a lot of `RQL.DML.Internal` was generic over the backend type, instead of being specialized to `'Postgres 'Vanilla`.
A consequence of this is that, before this PR, `DML.Internal` ended up having a dependency on non-Postgres backends, due to the use of `annBoolExp`, which requires a `BackendMetadata` instance. Since the code was written in a generic manner, `DML.Internal` in turn depended on having the metadata instances in scope... This PR changes that to, instead, explicitly import the Postgres instance.
(Note that this module didn't import `RQL.Types.Metadata.Instances`, but depends on a module that imports it, and **orphan instances are transitively imported**, as evidenced by the need for that explicit import in #4568.)
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4573
GitOrigin-RevId: 7b82b5d7c23c03654518a1816802d400f37c3c64
### Description
The main goal of this PR is, as stated, to remove the circular dependency in the schema building code. This cycle arises from the existence of remote relationships: when we build the schema for a source A, a remote relationship might force us to jump to the schema of a source B, or some remote schema. As a result, we end up having to do a dispatch from a "leaf" of the schema, similar to the one done at the root. In turn, this forces us to carry along in the schema a lot of information required for that dispatch, AND it forces us to import the instances in scope, creating an import loop.
As discussed in #4489, this PR implements the "dependency injection" solution: we pass to the schema a function to call to do the dispatch, and to get a generated field for a remote relationship. That way, this function can be chosen at the root level, and the leaves need not be aware of the overall context.
This PR grew a bit bigger than that, however; in an attempt to try and remove the `SourceCache` from the schema altogether, it changed a lot of functions across the schema building code, to thread along the `SourceInfo b` of the source being built. This avoids having to do cache lookups within a given source. A few cases remain, such as relay, that we might try to tackle in a subsequent PR.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4557
GitOrigin-RevId: 9388e48372877520a72a9fd1677005df9f7b2d72
### Description
There were several functions in `GraphQL.Schema.Common` that were unrelated to the schema building process, and were about metadata manipulation or dependency computation. Having those functions in the schema part of the code forces several places in the code to depend on the schema code, despite being completely unrelated.
This PR moves those functions where they make sense: alongside similar functions in `RQL.Types.*`, and rewrites `getRemoteDependencies` for clarity (it was using the term "indirect dependency" in a way that was inconsistent with the rest of the code).
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4568
GitOrigin-RevId: 948a18cebbb337a8bb6367c1f2d2ef5628209d96
### Description
Several places in the code used `a /= []`, which is inelegant. To my surprise, hlint did not warn about this, despite the fact that it forces an `Eq` instance on the elements. This PR replaces all occurrences of that pattern with `not (null a)` and adds a lint warning for it.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4569
GitOrigin-RevId: 6471e75ade9e71e5d583a0dac7815c01870c696b
By generalizing the instances, they can be written as attached instance derivations, rather than standalone ones.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4518
GitOrigin-RevId: 7a387911cf6ad46fe6acd36648275d6c2c68ffe3
A very minor cleanup (came out of documenting the architecture of actions). Does what's mentioned in the title.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4451
GitOrigin-RevId: d480ff438256df468df65b43d15f92a30b14b997
Previously, these were represented with a HashMap, but supposedly that map can never be empty. Now, it uses NEHashMap, which carries the non-empty invariant behind a smart constructor.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4481
GitOrigin-RevId: 93ad9aaa9354f25a1ba10e8207ae19614e1e439e
