Backport of Docs: GUI custom login feature into release/1.20.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #30799 to be assessed for backporting due to the inclusion of the label backport/1.20.x.

The below text is copied from the body of the original PR.

---

Description

Docs for feature merged by PR #30700

To view latest preview scroll all the way down and click the latest View deployment beside the Vercel bot below.

Most of the feature documentation is located at: /vault/docs/ui/custom-login but a tip has been added to supported auth methods as well, navigate to /vault/docs/auth/ldap to see in LDAP, for example:

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.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

---

Overview of commits

• 3701dcf

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅