docs: add llm-grok

[Hiepler]

Addsllm-grok xAI API (https://github.com/Hiepler/llm-grok) to the plugin directory.

[simonw]

Not sure why the tests were failing, this looks like it could be an intermittent GitHub Actions error unrelated to this change:

https://github.com/simonw/llm/actions/runs/11819065232/job/32959734138

Deleting the contents of '/home/runner/work/llm/llm'
Initializing the repository
Disabling automatic garbage collection
Setting up auth
Fetching the repository
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/patch-1*:refs/remotes/origin/patch-1* +refs/tags/patch-1*:refs/tags/patch-1*
  The process '/usr/bin/git' failed with exit code 1
  Waiting 18 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/patch-1*:refs/remotes/origin/patch-1* +refs/tags/patch-1*:refs/tags/patch-1*
  The process '/usr/bin/git' failed with exit code 1
  Waiting 17 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/patch-1*:refs/remotes/origin/patch-1* +refs/tags/patch-1*:refs/tags/patch-1*
  Error: The process '/usr/bin/git' failed with exit code 1