[imorland]

Changes proposed in this pull request:
Companion to flarum/framework#5439 ([2.x] feat(core): improve the standalone 404 page).

Document how themes and extensions add artwork and recovery links to the standalone 404 page in Flarum 2.1. Include the layout sections required by existing theme overrides.

Reviewers should focus on:
The partial override examples and their behaviour when search is unavailable.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.