### 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
