Move Technical Writing related handbooks to Product
@seanpackham I had to prepare the field to redact the comm writers program v2 (!5376 (merged)) in the right place.
Notes:
- The content wasn't altered, just moved around and links updated.
- Deletions from the Marketing Handbook: removed tech writing from dev rel.
- Additions to the main Handbook: tech writing and markdown guide under Product instead of Marketing.
cc/ @JobV @timanglade FYI