docs: publish via lance.org instead of this repo's GitHub Pages

[beinan]

Summary

Switches the docs from a standalone GitHub Pages site to lance.org, which is how the other subprojects publish.

I'd previously set up Pages here (#203, #206) and asked for the repo's Pages setting to be enabled. @jackye1995 pointed out that the org doesn't enable per-subproject Pages by design — public docs should build into lance.org. He's right, and it turns out the docs are already in exactly the right shape for it: lance.org is built from the lance repo, whose make-full-website.sh copies each subproject's docs/src into the main site.

So the content stays; only the publishing route changes.

Changes

• Add .pages nav files. The main site uses the awesome-pages plugin, so navigation must come from .pages files rather than a hardcoded nav: block — otherwise these pages render unordered under lance.org. Enabling the plugin here too keeps local preview and the published site on one source of truth.
• Drop the hardcoded nav: from mkdocs.yml, now redundant.
• site_url → https://lance.org/integrations/context/, the address the docs will actually be served from.
• Remove .github/workflows/docs.yml. It deployed to a Pages site that doesn't exist and can't be created without an org-level policy change, so it only ever produced a red X on main.

Verification

Ran lance/docs/make-full-website.sh against this checkout and built the full site: all 12 pages land under site/integrations/context/, and the nav nests correctly (Quickstart, Rollouts (RolloutDB), Rollout schema, ... all appear in the sidebar).

Depends on

lance-format/lance#8014 — until that merges, this repo's docs simply aren't picked up by the website build. Nothing here breaks in the meantime; mkdocs build --strict still passes locally for preview.

Follow-ups from the same conversation

Two other things Jack raised, not in this PR:

• Developer guide → CONTRIBUTING.md, which also auto-builds into lance.org. The lance-side PR already wires up the copy rule; this repo has no CONTRIBUTING.md yet, so that's a separate change.
• Discussions are moving to lance-format/lance/discussions org-wide.

Also worth cleaning up once this lands: the now-unused gh-pages branch on this repo.

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