Publish Operator docs to docs.gitlab.com

Summary

Context

The Operator docs under doc/ are not automatically included in https://docs.gitlab.com.

Problem

Changes to our documentation require two merge requests:

1. Operator MR to update doc/
2. Charts MR to update doc/installation/operator.md

This is redundant and error-prone.

Solution

Similar to Charts, include the Operator in the process that publishes to https://docs.gitlab.com.

Acceptance criteria

• Docs from Operator's doc/ directory are published to https://docs.gitlab.com without requiring a Charts MR
• The Charts documentation on Operator installation page content is replaced with content from the Operator repository