Expand userstyle docs

[WorldLanguages]

Some TODOs below. Some of these would belong in the _index.md file - while others should be considered best practice or debugging tips.

• Document addon settings CSS variables
• Examples on how to use customCssVariables (there's some info already on the manifest doc - probably needs its own page?)
• Document how addons should implement dark mode
• Document typical usages of userstyle.if in manifests
• Document updateUserstylesOnSettingsChange in more detail
• Write a small guide on z-index and the current values used by addons, specially editor ones
• Document browser support and CSS features that we can't use yet
• Document .sa-body-editor
• Document that CSS selectors should not target hashed class names
• More details on specificity, addon.json order, and usage of !important
• Document that certain languages can have longer strings for UI elements, so that needs to be accounted for in some cases
• Add examples on how to support RTL in the project editor
• Explain in detail the purpose of injectAsStyleElt and document %addon-self-dir%

[WorldLanguages]

@mxmou I'd appreciate if you could think of other things that are worth documenting. Also, feel free to edit the files directly on this branch, if you find yourself in a writing mood (I admit it's not easy to write docs "from scratch")

[WorldLanguages]

The current userstyle docs (master branch) are almost non-existent, so I'll merge this progress very soon.