docs: build & deploy pouchdb.com on GitHub Pages

[alxndrsn]

Add github action allowing manual building of the docs website from master, or any branch which includes .github/workflows/github-pages.yml with on: workflow_dispatch.

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The aim of this PR is to replace the current hosting of pouchdb.com on a private server with Github Pages, and allow build & deploy of docs via Github Actions.

DNS changes will be required to make pouchdb.com point to the Github Pages version.

Currently, www.pouchdb.com forwards to pouchdb.com. This may need additional work to be supported. See: https://stackoverflow.com/questions/54817253/github-pages-https-www-redirect.

Depending on how pages deployments will be restricted, environment branch protection rules must be configured. E.g. to allow deploys to pages from random branches/tags: Settings > Environments > github-pages > Deployment Branches > All Branches

This branch currently deploys to https://alxndrsn.github.io/pouchdb/, and updates can be triggered from https://github.com/alxndrsn/pouchdb/actions/workflows/github-pages.yml. Currently missing stylesheets and with links broken, as it assumes the hosting root is /, as it would be on pouchdb.com.

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Manual deploy will be available from the repo's Github Actions tab once the relevant action is in master. It should look like this:

Action View:

Run Workflow dialog:

[alxndrsn]

This PR needs to be updated to modify existing .github/workflows/docs.yml instead of adding a new GHA workflow.