Document CSS shape() relative control points and add related Fx146 rel notes

[chrisdavidmills]

Description

Firefox 146 adds support for CSS shape() relative control points, as used in <curve-command> and <smooth-command> commands, behind the layout.css.basic-shape-shape.enabled pref. See https://bugzilla.mozilla.org/show_bug.cgi?id=1921501, and in particular, this comment for some useful details.

This PR:

• Adds entries to the Firefox 146 rel notes and Experimental features pages to detail the new availability.
• Updates the shape() function reference page to improve the <curve-command> and <smooth-command> descriptions (making them easier to understand and adding details) and add some working examples.

Some notes:

• I don't think we need to update the BCD. The Chromium and Safari data appears to be correct (they already support relative control points), and the Firefox data is also correct (it is all still supported only in preview, behind a pref, even though that support arrived in different versions). Let me know if you disagree.
• Ideally, we should update this page to better organize the command descriptions and provide multiple examples of each of them in use. The function is really complex, so more guidance would be helpful. What I've done here is not perfect, but I think it does improve the coverage of the two commands in question, and allows me to link to them from the rel notes. I think a full page rewrite is out of scope for this PR.

Motivation

Additional details

Related issues and pull requests

[github-actions]

Preview URLs

• /en-US/docs/Mozilla/Firefox/Experimental_features
• /en-US/docs/Mozilla/Firefox/Releases/146
• /en-US/docs/Web/CSS/Reference/Values/basic-shape/shape

Flaws (183)

Note! 2 documents with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/CSS/Reference/Values/basic-shape/shape
Title: shape()
Flaw count: 183

• broken_links:
  • /en-US/docs/Web/CSS/fill-rule is a redirect
  • /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#question_mark is a redirect
  • /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#hash_mark is a redirect
  • /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#single_bar is a redirect
  • /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#single_bar is a redirect
  • and 174 more flaws omitted
• macros:
  • Macro produces link /en-US/docs/Web/CSS/position which is a redirect
  • Macro produces link /en-US/docs/Web/CSS/position which is a redirect
  • Can't resolve /en-US/docs/Web/CSS/Guides/Logical_properties_and_values/Houdini
• unknown:
  • page(Doc) not found /en-US/docs/Web/CSS/Guides/Scroll_driven_animations

External URLs (1)

URL: /en-US/docs/Mozilla/Firefox/Experimental_features
Title: Experimental features in Firefox

• https://bugzil.la/1921501 (1 time) (Note! This may be a new URL 👀)