fix(baseline): add custom elements group to blocklist

[LeoMcA]

Summary

Fixes:

• Baseline widget incorrectly indicates "Limited availability" content#32138
• CSS :defined says "limited availability" despite meeting Baseline requirements according to browser support table content#32008
• WRONG: wrong warning for CSS ::part(). It warns "Limited availability", but actually not. content#31932
• CustomElementRegistry.define() "This feature is not Baseline because it does not work in some of the most widely-used browsers." content#31764
• Baseline component seems to show incorrect Baseline status or BCD is incorrect content#31566
• CSS :defined - Appears to be discrepancy between Baseline status and support matrix web-platform-dx/web-features#562
• CustomElementRegistry.define() "This feature is not Baseline because it does not work in some of the most widely-used browsers." web-platform-dx/web-features#561

See also:

• Update Baseline status for custom elements web-platform-dx/web-features#571
• Baseline status for single features vs aggregated group status web-platform-dx/web-features#495
• Baseline component seems to show incorrect Baseline status or BCD is incorrect #10297

Problem

Much like #10345, users are confused by the overall custom elements "not baseline" banner showing on pages describing elements of custom elements which are interoperable cross-browser.

Solution

Hide these banners until we find a solution upstream.

How did you test this change?

Visited:

• http://localhost:3000/en-US/docs/Web/API/CustomElementRegistry/define
• http://localhost:3000/en-US/docs/Web/CSS/::part
• http://localhost:3000/en-US/docs/Web/CSS/:defined
• etc.