Nicer URL structure

[sampsyo]

I would like to simplify the URL structure for finding extensions and instructions in the manual. Namely, I propose this URL hierarchy:

http://riscv.fyi/M        # For an extension, listing all the instructions in that extension.
http://riscv.fyi/M/clmul  # For a specific instruction.

In other words, there should be top-level "directory" named for each instruction, like I, M, or Zifencei. Then, instructions are nested within the directory for the extension they belong to.