[AG-54778] KB add info about :remove-attr() and :remove-class() pseudo-classes

[techi9]

Documents the new uBlock Origin-compatible Extended CSS pseudo-classes :remove-attr() and :remove-class() in docs/general/ad-filtering/create-own-filters.md:

• added both pseudo-classes to the table of contents;
• added them to the list of pseudo-classes that have their own limitations;
• added full sections with syntax, examples and compatibility notes, modeled after the existing :remove() and :empty-trimmed sections.

The pseudo-classes are implemented in ExtendedCss v2.3.0 (AdguardTeam/ExtendedCss#153) and are consumed by AdGuard Browser Extension v5.6 (AdGuardSoftwareLimited/ext-tsurlfilter#78).

Kept as a draft until the AdGuard Browser Extension v5.6 release, so that the "supported by AdGuard Browser Extension v5.6 or later" note becomes true.

Testing instructions

• Open the preview of the page and check that the two new sections render correctly: headings, anchors, adblock code blocks and the compatibility notes.
• Verify the in-page links from the table of contents and from the limitations list jump to the new sections.
• Verify the :remove-attr() / :remove-class() links lead to the uBlock Origin static filter syntax page.

[github-actions]

Preview was deployed to: https://pull-request-948.kb-adg.pages.dev/