Move pkgdown building and deployment to GH action

[mojaveazure]

Automatically build and deploy pkgdown website via GH actions. New pkgdown.yaml CI workflow to build and deploy on commit to main or a GH release. Deployment occurs via a new gh-pages branch

Also, remove the existing pkgdown website from the main branch, and add the docs/ directory to .gitignore to prevent it from being re-checked in to main

SC-62380
resolves #798

[johnkerl]

LGTM!!

[jdblischak]

@mojaveazure awesome! Great work.

Good news: it's working! The site has been successfully deployed

https://tiledb-inc.github.io/TileDB-R/
https://github.com/TileDB-Inc/TileDB-R/commits/gh-pages/

Bad news: It looks like GitHub is still trying to deploy a site from main and hence this failed CI

https://github.com/TileDB-Inc/TileDB-R/actions/runs/12956945801/job/36144314412

My guess is that maybe the Pages Settings weren't updated in time. Hopefully the failed unnecessary run goes away with the next merge to main, but if not we'll have to investigate further.

[mojaveazure]

@jdblischak Correct about the main failure. I had merged this PR before updating the GH pages setting for the repo, so it tried and failed to deploy from main; however that should no longer be the case as the subsequent runs of pages-build-and-deploy have all been from the gh-pages branch (with run 417 in particular deploying the dev docs from built from main https://github.com/TileDB-Inc/TileDB-R/actions/runs/12956946072/job/36144315392 and hosted on gh-pages https://github.com/TileDB-Inc/TileDB-R/actions/runs/12957026566)

I'm going to try to land another PR to main in the near future and that should show that we're no longer deploying directly from main