[SVG] Support q units for SVG lengths

[bsmth]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Features to document

q units supported:

-  static const float CSSAbsoluteUnitConversionFactors[6][6] = {
-      // columns: px, cm, mm, in, pt, pc
+  static const float CSSAbsoluteUnitConversionFactors[7][7] = {
+      // columns: px, cm, mm, in, pt, pc, q

See WPT

• https://wpt.fyi/results/svg/types/scripted/SVGLength-px.html?label=experimental&label=master&aligned (SVGLength, converting from 'px' to other units (detached), q)

See also https://chromestatus.com/feature/5983903483428864

1q is equivalent to 1/40th of 1cm.

Related Gecko bugs

• https://bugzilla.mozilla.org/show_bug.cgi?id=1836995

Other

• Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.
• Check if glossary updates are required for the feature you're documenting - whether an existing term needs to be updated or a new term should be added.
• Check if BCD update means that content pages need to have experimental markup removed or deprecated markup added (front matter tags and macros).

Related issues and pull requests:

• Relnote: relnote(116): SVG q unit supported (defined in CSS length units) #27950

[bsmth]

As stated in the bugzilla bug, we are going to take a look at improvements for SVG docs in future as there are some areas where we are targeting SVG1.1 and others where we are documenting SVG2 functionality. We may split some of the content in https://developer.mozilla.org/en-US/docs/Web/SVG/Content_type out into separate pages where we can be more granular and add specification links independently for each type and highlight notable differences in spec versions.