docs(mcp-server): fix installation.md wording, heading levels, and link

- Nest step 3 content under the ordered list in the Docker image section
- Document the missing --interactive flag in the configuration options table
- Demote public instance, tool configuration, and authentication headings to h3
- Replace the absolute creating-oauth-app URL with a relative doc link
- Fix capitalization, trailing whitespace, and dash-vs-colon phrasing