Add .md files and llms.txt for AI agents

[gagdiez]

Following stripe's blog we want any URL to be easily transformed into their .md counterpart by simply adding ".md" to the URL.

For example, we would want to access the docs at https://docs.near.org/tools/near-api by simply navigating to https://docs.near.org/tools/near-api.md.

After, all such .md files should be referred to by https://docs.near.org/llms.txt.

[walterthesmart]

@gagdiez, I want to be assigned this issue.

[gagdiez]

#2632
#2624