docs(api-usage): add an anonymous example against a hosted sample GitLab API - #3466

Changes

Adds one more anonymous example to the gitlab.Gitlab code block in
docs/api-usage.rst: gitlab.Gitlab('https://gl.sandboxapis.dev').

Disclosure first: we run that service. It is SandboxAPIs (Driftwork LLC), a
hosted, read-only copy of the GitLab v4 REST API filled with sample projects,
merge requests and users, and it answers requests without a token. People
regularly ask how to try python-gitlab code without a GitLab account or a
token they are allowed to use, and this is one answer to that. It is not
affiliated with GitLab.

If you would rather not have a commercial service named in the docs, please
close this; no follow-up from us. If you would prefer it worded differently
or placed elsewhere (the FAQ, for example), say where and we will change it.

Checked with python-gitlab 8.6.0 on Python 3.12, no token:

import gitlab
gl = gitlab.Gitlab("https://gl.sandboxapis.dev")
project = gl.projects.get("olympus-labs/parthenon")
print(project.path_with_namespace, project.default_branch)
# olympus-labs/parthenon main

Documentation and testing

• Documentation in the matching docs section
• Unit tests and/or functional tests (docs-only change)

This PR was prepared with AI assistance (Claude Code), reviewed by a person
before posting.

🤖 Generated with Claude Code