add Markdown media type

[elocremarc]

Markdown is a desirable media type to add now that we are inscribing Libraries.
This would be a good way to document code onchain.
You could keep the documentation in the same UTXO as the library or create a child parent using the library inscription
Clients would probably need to create a recursive markdown parser to add links and images for instance.
[link](<inscriptionId>)
![Image](<inscriptionId>)
People could even make clients like obsidian using the same recursion above.
This would also allow people to easily write a blog on inscriptions.

[raphjaph]

With some markdown renderer as recursive inscription this could work nicely.