docs: add prompts partials documentation and examples

[kushalshit27]

🔧 Changes

Documents the partials feature under the prompts resource, which allows injecting custom HTML (via Liquid templates) into specific insertion points of a prompt screen.

Examples

YAML format

Inline Liquid content in tenant.yaml:

prompts:
  identifier_first: false
  universal_login_experience: new
  webauthn_platform_first_factor: false
  partials:
    login:
      login:
        form-content-start: |
          <div class="login-notice">
            Welcome back! Please log in to continue.
          </div>
    signup:
      signup:
        form-content-start: |
          <div class="signup-notice">
            Create your account to get started.
          </div>
        form-content-end: |
          <p class="signup-terms">
            By signing up, you agree to our <a href="/terms">Terms of Service</a>.
          </p>

Directory format

Folder structure:

./prompts/
    /partials
        /login
            /login
                /form-content-start.liquid
        /signup
            /signup
                /form-content-start.liquid
                /form-content-end.liquid
    /partials.json

partials.json manifest (paths relative to prompts/):

{
  "login": [
    {
      "login": [
        {
          "name": "form-content-start",
          "template": "partials/login/login/form-content-start.liquid"
        }
      ]
    }
  ],
  "signup": [
    {
      "signup": [
        {
          "name": "form-content-start",
          "template": "partials/signup/signup/form-content-start.liquid"
        },
        {
          "name": "form-content-end",
          "template": "partials/signup/signup/form-content-end.liquid"
        }
      ]
    }
  ]
}

🔬 Testing

• No source-code changes were made in this PR

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.15%. Comparing base (3c8d3f1) to head (9cf12f4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1318   +/-   ##
=======================================
  Coverage   80.15%   80.15%           
=======================================
  Files         152      152           
  Lines        6096     6096           
  Branches     1245     1245           
=======================================
  Hits         4886     4886           
  Misses        693      693           
  Partials      517      517           

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