Move Momentum docs to the private momentum-docs submodule

[juliebin]

Part of SparkPost/Momentum#1765.

Background
Every merge to this repository publishes to docs.sparkpost.com. Momentum is on-premises, and its docs must not describe a feature before that release ships, so Momentum doc PRs have had to wait unmerged until release time.

Change

• content/momentum/ becomes a git submodule of the private repository SparkPost/momentum-docs. It is pinned to a commit with the same content as main today, so the published site does not change. Momentum doc changes now merge in momentum-docs; publishing a release means moving this submodule to the release's tag.
• netlify.toml sets the build command npm run build, the same command as the Netlify UI.
• The Cypress, Lighthouse and Algolia index workflows fetch the submodule with a read-only deploy key, through the new .github/actions/momentum-docs action and the secret MOMENTUM_DOCS_SSH_KEY.
• The Momentum search index name can be set with NEXT_PUBLIC_ALGOLIA_MOMENTUM_INDEX, so momentum-docs previews can search their own index. When it is unset, the site and the indexer use next_momentum_documentation as before.
• pdf-build reads the manuals through the submodule, and stops with an error if the submodule has not been fetched.

Known costs

• Pull requests from forks now fail the Cypress and Lighthouse checks, because GitHub gives them no secrets and they cannot fetch the private submodule.
• A local clone needs git submodule update --init before npm run dev or npm run build, which needs read access to momentum-docs.

Testing

• I built this branch and main side by side. Both produced the same 3,438 pages, and the remaining byte-level differences were build-to-build noise.
• This PR's Netlify Deploy Preview is the first build that fetches the submodule with Netlify's deploy key.

🤖 Generated with Claude Code

[netlify]

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