CSS Module: syntax (and error handling guide)

[estelle]

The original task was to update the charset module. Turns out there is no such thing.
Charset was defined in CSS 2 but removed in the CSS syntax module.

• Move css_charset to css_syntax.
• Created a new document: css_syntax. This is a module landing page. It includes the key concepts introduced in that module.
• Created a new document: a guide on CSS error handling by the parser. a subpage for CSS syntax.
• Updated the @charset page to indicate that it is NOT an at-rule, and has been deprecated in css-syntax.
• Updated custom-ident, the page that had the best explanation of CSS escape characters, adding a separate section for escaping characters in CSS. This PR does not include creating links to that section. That will be done as a separate PR (this PR is already big enough)

related to openwebdocs/project#186

This PR was previously reviewed and is now co-authored by @dipikabh. See #30793

[github-actions]

Preview URLs

• /en-US/docs/Web/CSS/@charset
• /en-US/docs/Web/CSS/CSS_syntax/Error_handling
• /en-US/docs/Web/CSS/CSS_syntax
• /en-US/docs/Web/CSS/custom-ident

External URLs (1)

URL: /en-US/docs/Web/CSS/custom-ident
Title: <custom-ident>

• https://en.wikipedia.org/wiki/Case_sensitivity (2 times) (Note! This may be a new URL 👀)

(comment last updated: 2023-12-15 08:29:31)