Add llms.txt to developer docs

[kaankacar]

Other ecosystem's developer docs have llms.txt

• https://docs.near.org/llms.txt
• https://solana.com/llms.txt

Mvx docs could benefit from having this as well. Some info on llms.txt: https://llmstxt.org/

Will work on this myself, created this issue to keep track of this contribution