Migrate more pages to specs

[zzz]

str4d migrated most of our important specs to .rst markdown in the specs/ directory but stopped long before it was complete.

Several pages remain in the api/, how/, protocol/, and transport/ directories that are actually formal specs. Some others may or may not be, but it's odd that those pages are separated from the spec portion in spec/.

In particular, elgamal-aes, BOB, SAM, i2pcontrol, I2CP, I2NP, network-database, NTCP, and SSU pages should probably be migrated. With increased interest from Monero on restarting router development, we should get our specs in order. Opening this ticket as a reminder that this task is not complete.

I don't think everything under the docs/ part of the tree is a spec and needs to be migrated, but a lot of it is.