Move hexadecimal values to new <hex-color> page

[teoli2003]

Per #15540 (comment)

(I have not renamed the bcd key as I want to be sure this will work before again updating bcd. Meanwhile, it works here, and the mdn_url in bcd leads to a redirect, but works)

I have not yet updated pages linking to this one (not many); I'll do this in a follow-up.

[github-actions]

Preview URLs

• https://pr15674.content.dev.mdn.mozit.cloud/en-US/docs/Web/CSS/hex-color

Flaws

URL: /en-US/docs/Web/CSS/hex-color
Title: <hex-color>
on GitHub
Flaw count: 1

• bad_bcd_links:
  • no explanation!

External URLs

URL: /en-US/docs/Web/CSS/hex-color
Title: <hex-color>
on GitHub

No new external URLs

[wbamberg]

Thanks @teoli2003 , I think this looks great!