PKI SCEP documentation updates

[stevendpclark]

Description

This adds the relevant updates for the PKI SCEP feature. Adding documentation around the new SCEP auth plugin along with the new protocol paths and configuration API within the PKI mount.

VAULT-27802

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
  • LTS: If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
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  description. Also, make sure the changelog is in this PR, not in your ENT PR.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[victorr]

Nice work!

Reviewing with request changes as the nested bullet points need fixing.

Also left many suggestions, please discard any of them that you don't agree with.

[victorr]

Awesomesauce.

[kitography]

Double but sentance construction here is a little confusing, maybe:

"This endpoint is used to authenticate against the SCEP auth method. It shouldn't be used directly, rather through delegated authentication from a PKI mount."

[stevendpclark]

Fixed.