Handbook updates for docs as SSOT / docs-first: New Documentation section, updated Technical Writing, Support

• Add /handbook/documentation - inspired by the need to add clarity about important docs topics such as 'docs as product SSOT' and 'docs-first methodology' (per #3578 (closed)). (/handbook/product/technical-writing already exists but we can make that section more about the team, its practices/processes, how to work with it, etc.)

Given the importance of Documentation as a topic - and having clarity on that topic among the GitLab community - it makes sense to have this Documentation page/section, which offers key information and then links out to various, disparate resources including:

• the section on contributing to documentation within the documentation itself (/development/documentation)
• the markdown guide for docs (currently in the product/technical-writing handbook section and should probably be moved)
• the GitLab Docs repo

Also:

• Rewrite /handbook/product/technical-writing/index.md
• Add Support role in docs to the Support Handbook section