Add Google PSE exported configuration and docs

What does this MR do and why?

• Adds Google Programmable Search Engine configuration to the gitlab-docs codebase.
• Adds a new CODEOWNERS entry for these files.
• Adds documentation to define a config change process.
• Adds documentation about modifying synonyms.xml (the most likely config change a TW might want to do).

Why

• Having exported configuration would allow us to more easily recreate the Programmable Search engine if needed.
• Tracking config changes in code gives us a historical record of modifications.

Caveats

• There is not a way to automate config deployment (or retrieval); it has to be done from the Google PSE dashboard, which is only accessible to team members who have been added by another admin (currently that's myself, Susan, axil, and a person from the GitLab SRE team who originally spun up the service).
• Since uploading new config is manual, it's possible for configuration between Google and these exported files to fall out of sync if we merge a change but neglect to upload the new file to Google (or make a change on Google without updating the exported XML).

I do not expect configuration to change very often, so I think the trade-offs are OK here, and it's still worth tracking these files in Git. Open for discussion though! 😄

Closes #1590 (closed)

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