Move webext

[ffoodd]

New repository: a11y.css-webextension.

Closes #436

To-do

• [x] webextension's doc page calls a11y-en_webextension-only.css, which does not exist
• [x] add link to new webext repo in README, wiki and docs
• [x] ~create an organization for a11y.css?~ — not sure it'd be useful…
• [x] redo #447 in new repo
• [x] Move WebExtension's wiki to the new repo (in both languages, obviously) [WIP]
  • [x] Développement à traduire en FR
  • ~Traduire les commentaires dans les blocs de code, ou pas~
  • ~Refaire une navigation~
• [x] Update stylelint config
• [x] Enhance Sass workflow: ensure each CSS file is generated correctly from Sass, mostly

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