DM-55634: Backport the Edit on GitHub source-directory fix to 2.4

[jonathansick]

Summary

Backport of #352 onto the new 2.4 release branch, for a 2.4.x patch release.

• The "Edit on GitHub" button now resolves its in-repository path from the Sphinx source directory rather than the directory holding conf.py. Projects that build with sphinx-build -c . docs _build/html (where those differ) no longer emit links that 404 — this is what https://dp2.lsst.io/ hits today. A source directory at the repository root also no longer produces a stray ./.
• The path is computed by a new documenteer.ext.githubeditlink extension at config-inited, the earliest event where srcdir is known. An explicitly configured html_context["doc_path"] is honored and short-circuits auto-detection.
• Building outside a Git checkout no longer raises InvalidGitRepositoryError at configuration time; the button is omitted instead.

Backport adaptations

The 2.4 branch predates several things the original PR touched, so this is not a clean replay:

• _toml.py drops its ._utils import entirely rather than narrowing it — normalize_origin_base_url and the storage clients don't exist on 2.4.
• The test-guide and test-linkcheck-service* roots don't exist yet, so only test-redoc is relaxed (and gains a github_url, since show_github_edit_link now defaults on).
• tests/test_conf_toml.py takes only the three set_edit_on_github guard tests; the link-check service cases belong to a later feature.
• tests/ext/conftest.py is brought along, because tests/roots/test-redoc also builds the guide preset — without the module-eviction fixture, the two guide-stack builds in one pytest process would share cached settings.

Validation steps

Run against this branch's own tooling (2.4 uses tox, not nox):

tox -e py-test-sphinx8    # 84 passed
tox -e typing-sphinx8     # clean
tox -e lint               # clean
tox -e docs               # builds, including the new extension page

All four pass locally. The suite includes the wiring test that drives the real guide preset in the -c ./docs layout, which was confirmed on main to fail if the extension is unregistered.

References

• Backport of DM-55634: Fix the Edit on GitHub link's document path #352
• Jira: https://rubinobs.atlassian.net/browse/DM-55634