chore(ci): add deploy docs workflow

[katebatura]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Adds deploy-docs workflow which deploys docs on every master commit.
Updates build-docs script to reuse a built docs app from gh-pages branch instead of building the app again.