Migrate remaining Unboxed repo modules into courses

Fix ContentLayer warnings https://github.com/contentlayerdev/contentlayer/issues/495
Moved course-structure from https://github.com/Unboxed-Software/solana-course/pull/384 to this repo using metadata.yml files
- The "extending-solana-functionality" section was split into "connecting to offchain data" course (with oracles and vrf) and a separate "solana pay" single-lesson course.
- 'versioned-transaction' was renamed to 'lookup-tables' (this content is focused on overcoming size limits by lookup tables rather than on versioned transactions per se)
- 'front end for native' lessons were migrated into the 'native development' course
- 'env-variables' was renamed to 'program configuration' (as the content is focused on multiple methods for program configuration)
- 'cpi' and 'vrf' renamed to 'cross-program-invocations' and 'verifiable-randomess-functions' to match the naming convention of the other lessons
- Tweak the Anchor CPI text to avoid assuming people have read the native CPI lesson.
- Re-add burning and delegation exercises in a new lesson and lab
Prioritize courses, so simpler ones are shown first
Fix internal relative links to the other lessons
Added images and fixed image links
Changed any link text that includes '//' to work around an apparent bug in https://github.com/remarkjs/remark-gfm
Add slashes to escape < and { in Markdown tables that stop the markdown parser.
Add descriptions for all lessons (and ensure descriptions fit in UI for solana.com)
Use Callouts for tips
Remove a few remaining instances of old airtable form

solana-foundation / developer-content

Migrate remaining Unboxed repo modules into courses #250