Add initial documentation

[kmaziarz]

This PR adds an initial static documentation built using Material for MkDocs. The docs are built automatically using GitHub Actions and deployed to microsoft.github.io/syntheseus/.

This first version is a combination of content we had in README.md (which is changed here to be very minimalistic, linking to the docs for further information) and the pre-existing docs/cli/eval_single_step.md. In future PRs we can extend the docs with documentation for the search CLI, as well as an API reference and tutorials.

[kmaziarz]

Note that rebuilding of the docs happens either when a PR is merged into main or when one triggers it manually from the Actions tab. This means that, when testing a PR that changes the docs, running the action would also publish the changes to be externally visible before the PR is actually merged (although this would only be the case for the final test, as mkdocs can serve the work-in-progress locally; the result of running the action can also be easily reverted by re-running the action from main). This setup seems fine for now, although if we want we could consider enabling some sort of versioning in the future (likely there is a good solution provided by Materials for MkDocs, I just haven't looked into that yet).

[AustinT]

Looks fantastic Kris, thanks for pushing this forward!