[Meta] 2.4

[pascalduez]

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WIP

• [x] Poster annotations override #387, #410
• [x] Docs for poster annotations override SassDoc/sassdoc.github.io#120
• [ ] Try to infer default values when compiling #304
• [x] Update linting #400, #414, #447
• [x] Colour variables references sassdoc/sassdoc-theme-default#78
• [x] Docs for sassdoc-extras resolved values (2.4.0) SassDoc/sassdoc.github.io#123
• [ ] Document CSS context parsing SassDoc/sassdoc.github.io#126
• [ ] Add annotation aliases unit tests #446
• [ ] ...