Add document about the v2 reference-generation process

[nbogie]

Addresses #8508

Changes:

Add contributor_docs/reference_generation_process.md

• section on main reference generation process
  • flow diagram
  • description of process steps and important files
• section on type-generation
  • sparse description (lists main flow and processes)
  • flow diagram
  • description of the patch process
• section (placeholder) on search index generation (delete?)

(WIP at the moment. Just moving out from #8462 so that that can focus on the inline-reference guide)