[imorland]

Fixes #0000 (no issue: documents flarum/framework#4570, which fixes flarum/framework#4566)

Changes proposed in this pull request:
Documents the English fallback for gambit keywords that flarum/framework#4570 adds in Flarum 2.1. search.md gets a "Localizing gambit keywords" section on canonicalKey(), mapping matched keywords to stable filter values, and translating keywords; language-packs.md points language pack maintainers to it; and the 2.1 section of the 2.x upgrade guide tells extension authors to override canonicalKey() when their key() is translated.

Reviewers should focus on:

• Merge after [2.x] feat(gambits): localization alias support — English keywords always work regardless of locale framework#4570, since this describes its API.

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.