docs: Which table goes "flags" under? (#1452)

- There is currently no indication under which [table](https://toml.io/en/v1.0.0#table) do "flags" go in the config file.
- I think having that being explicitly spelled out & having an example saves quite a bit of people's time. Would for me 🙃

docs/content/configuration/config-file/flags.md

@@ -4,6 +4,13 @@
 
     This section is in progress, and is just copied from the old documentation.
 
+You can configure flags by putting them in `[flags]` table. Example:
+
+```toml
+[flags]
+color = "nord-light"
+```
+
 Most of the [command line flags](../command-line-options.md) have config file equivalents to avoid having to type them out
 each time: