bsn: improve docs and add tests

[laundmo]

Objective

Improve documentation of bevy_scene/bsn.

Rendered: https://share.yadamiel.com/docs/doc/bevy/

Solution

This PR depends on/includes changes from

• bsn: fix accidental scene entity deduplication due to name references #24402

Docs changes (approx)

• removing confusing Inheritance concept in favour of caching
  • BSN: changing prefixes for Template and SceneComponent to allow removing confusing "inheritance" concept #24367
• fix now-changed syntax
  • BSN: changing prefixes for Template and SceneComponent to allow removing confusing "inheritance" concept #24367
• cleaning up the duplication between macro docs and module docs
• clean up the syntax snippet to be a single table
• a bunch of smaller fixes

Testing

• cargo doc -Zrustdoc-mergeable-info -p bevy_scene --lib locally works fine
• the other cargo doc command also works fine (see cargo.toml diff for full command, its ugh)

Todo

• show how scenes actually get evaluated, not a full expansion but a rough idea of it
• come up with something better for the macro inline docs than a complex example macro call
• probably remove the bsn: fix accidental scene entity deduplication due to name references #24402 commits again, they're annoying to have in the history

[laundmo]

Note to self: somehow mention users can implement it