switch GitHub pages to gh-pages branch

[willholley]

What this PR does / why we need it:

This updates .asf.yaml to use the gh-pages branch as the source for GitHub pages (published to https://apache.github.io/couchdb-helm). This effectively migrates the Helm repository to that branch which defines a new index.yaml referencing the newly created GitHub Release artifacts (see #79).

In addition:

• Require linear history for the main branch to ensure PRs are up to date prior to merge.
• Add branch protection to gh-pages to prevent force pushes.

The remaining steps to complete migration are:

1. Verify the new gh-pages content/Helm repository.
2. Remove the legacy /docs folder.
3. Update the PR template to reflect that manual chart packaging is no longer required.
4. Update CONTRIBUTORS.md to relfect the new process for chart publishing.

[colearendt]

Nice!

Just a note for future reference that this migration means:

• tarballs will no longer be stored in the repository (i.e. the gh-pages branch is basically just for hosting the index.yaml file for the repository)
• tarballs will be stored in GitHub Releases, and referenced by the index.yaml file using the conventions from the helm community

[colearendt]

FWIW it is possible to test this new repo without the migration:

helm repo add couchdb-test https://raw.githubusercontent.com/apache/couchdb-helm/gh-pages/
# "couchdb-test" has been added to your repositories
helm search repo couchdb-test
# NAME                	CHART VERSION	APP VERSION	DESCRIPTION
# couchdb-test/couchdb	3.6.1        	3.2.1      	A database featuring seamless multi-master sync...