Add page on integrity; revise SRI guide

[wbamberg]

Fixes #43302.

We didn't have a standalone page for integrity, and since it's a bit involved and used in more than one element. I thought it was worth creating one.

I've talked about including multiple hash values, and the behavior as I understand it.

TBH I don't understand why anyone would provide more than one hash function rather than just use SHA-512. Maybe I'm missing something?

I haven't systematically rewritten the SRI guide, just made some changes that seemed necessary.

I would be tempted to remove a lot of the detail about "how to generate hashes" and just point people at tools like OpenSSL.

[github-actions]

Preview URLs (5 pages)

• /en-US/docs/Web/HTML/Reference/Attributes/integrity
• /en-US/docs/Web/HTML/Reference/Attributes
• /en-US/docs/Web/HTML/Reference/Elements/link
• /en-US/docs/Web/HTML/Reference/Elements/script
• /en-US/docs/Web/Security/Defenses/Subresource_Integrity

Flaws (34)

Note! 4 documents with no flaws that don't need to be listed. 🎉

Found an unexpected or unresolvable flaw? Please report it here.

URL: /en-US/docs/Web/HTML/Reference/Attributes
Title: HTML attribute reference
Flaw count: 34

• broken_links:
  • Link /en-US/docs/Web/HTML/Reference/Attributes/alt doesn't resolve
  • Link /en-US/docs/Web/HTML/Reference/Attributes/autoplay doesn't resolve
  • Link /en-US/docs/Web/HTML/Reference/Attributes/cite doesn't resolve
  • Link /en-US/docs/Web/HTML/Reference/Attributes/colspan doesn't resolve
  • Link /en-US/docs/Web/HTML/Reference/Attributes/controls doesn't resolve
  • and 29 more flaws omitted