docs: update pluggable HTTP client guide to use OkHttpGitHubConnector

[SAY-5]

Description

Fixes #2126.

The "Pluggable HTTP client" section of src/site/apt/index.apt referenced the legacy OkHttpConnector plus OkUrlFactory.setCache() API, which no longer match the current OkHttpGitHubConnector (in org.kohsuke.github.extras.okhttp3). New users following this guide could not find a working integration path; the maintainer note on the issue confirms the desired direction (use OkHttpGitHubConnector + GitHubBuilder.withConnector()).

The sample now:

• references GitHubConnector (the actual interface) instead of HttpConnector,
• names OkHttpGitHubConnector and its package so it is greppable,
• builds the OkHttpClient via OkHttpClient.Builder().cache(...), matching the in-tree GitHubCachingTest.

Before submitting a PR:

• Changes must not break binary backwards compatibility.
• Add JavaDocs and other comments explaining the behavior.
• When adding or updating methods that fetch entities, add @link JavaDoc entries to the relevant documentation on https://docs.github.com/en/rest .
• Add tests that cover any added or changed code. (Docs-only change; no behavior change.)
• Run mvn -D enable-ci clean install site "-Dsurefire.argLine=--add-opens java.base/java.net=ALL-UNNAMED" locally. (Spotless verified locally.)
• Push your changes to a branch other than main.

When creating a PR:

• Fill in the "Description" above with clear summary of the changes.
  • If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • Provide links to relevant documentation on https://docs.github.com/en/rest where possible.
• All lines of new code should be covered by tests as reported by code coverage. (Docs-only.)
• Enable "Allow edits from maintainers".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.30%. Comparing base (3622553) to head (27446ec).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2266   +/-   ##
=========================================
  Coverage     85.30%   85.30%           
  Complexity     2553     2553           
=========================================
  Files           241      241           
  Lines          7540     7540           
  Branches        396      396           
=========================================
  Hits           6432     6432           
  Misses          872      872           
  Partials        236      236           

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