Documentation - Merge Mintlify docs and replace legacy docs

[MantisClone]

Goal

Make the Mintlify docs the public Request Network docs and deprecate the legacy GitBook docs.

Sprint scope

Cutover first. This issue is intentionally limited to:

• Review the current Mintlify preview / PR stack and merge it once acceptable
• Fix only merge/cutover blockers needed to make the new docs usable as the primary docs
• Point docs.request.network to the Mintlify docs
• Move the current GitBook docs to legacy.docs.request.network
• Update obvious legacy-doc references required for the cutover

Out of scope for this issue

• Adding new product documentation
• Expanding self-onboarding content from Notion/PDF
• Polishing every migrated page before launch

Those follow after the new docs replace the old site.