ci: docs build check on PRs + consolidate PyPI publishing

[romeokienzler]

Two release-process hardening changes, following the v0.5.0 release where a stale docs reference broke the deploy and two PyPI workflows raced.

1. Catch docs breakage on PRs (docs-check.yml)

New workflow runs mkdocs build --strict on pull requests that touch docs/**, mkdocs.yml, src/**, or terratorch_iterate/**. The v0.5.0 docs deploy failed because mkdocstrings ::: directives still pointed at the renamed benchmark package — --strict would have caught that (and dead nav links) on the PR. Installs the same deps as the deploy + pip install -e . for introspection.

2. Consolidate PyPI publishing

Previously both publish-pypi.yml (on tag v*) and python-publish.yml (on release published) fired for every release and raced to upload identical artifacts — the loser logged a 400 Bad Request.

• Keep the more robust tag-based publish-pypi.yml (has twine check, setuptools_scm, fetch-depth: 0).
• Add --skip-existing to the upload so re-triggering a tag (e.g. moving it to pick up a docs fix, as done for v0.5.0) no longer fails on the already-published version.
• Delete the redundant python-publish.yml.

Note: publishing now triggers on the tag, not the GitHub Release event. The normal flow (create a release, which creates the tag) still publishes. If a release is ever created from a pre-existing tag, no tag-push event fires — use the tag to drive releases, as the existing process does.

Verified mkdocs build --strict exits 0 locally.

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