[imorland]

Fixes #0000 (no issue: documents flarum/framework#5199)

Changes proposed in this pull request:
Documents the icon picker that flarum/framework#5199 adds in Flarum 2.1. fontawesome.md gets a "Choosing an Icon" section: what the picker lists for each Font Awesome source, how to use an icon it doesn't list, and that assets on another domain need Access-Control-Allow-Origin for icons to be listed. The tags page's Icon field points to it. For extension developers, admin.md adds the icon setting type, forms.md the IconInput component, and code-splitting.md notes that IconPicker is a lazy core module.

Reviewers should focus on:

• Merge after feat: add a reusable Font Awesome icon picker framework#5199, since this describes it, and not before Flarum 2.1 is released.

Screenshot
N/A

Necessity

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

Confirmed

• Frontend changes: tested on a local Flarum installation. (No code changes.)
• Frontend changes: tests are green (run yarn test in js/). (No code changes.)
• Frontend changes: tests have been added, or are not appropriate here. (No code changes.)
• Backend changes: tests are green (run composer test). (No code changes.)
• Backend changes: tests have been added, or are not appropriate here. (No code changes.)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (N/A)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.