[docs] Refresh add_ops.md

[bjacobgordon]

• removes section regarding Turbine Camp
  • Each line of detail either:
    • already existed internally in Confluence
      OR
    • was severely out of date!
  • Was beyond the concerns of Torch-MLIR
• adjusts link to LLVM style guide
  • was directed to a specific style guide rule rather than the start of the style guide in general
• adds missing h2
• cleans up style using markdown linter
  • prefers formatted links over intentionally bare URLs
  • enforces explicitly-define language in code blocks
  • prefers implicitly-ordered, 1-based lists
    • avoids less-common 0-based lists since that would require deviation from the default linter config
  • wraps bare urls/emails
  • enforces unordered list nested indentation
  • enforces space around headers
  • enforces space around code fence blocks
  • removes extraneous blank lines
  • enforces space around list blocks

[bjacobgordon]

Cleaned up this doc while going through Turbine Camp!
 
Commits are "atomic" as follows:

• style commits are pure linting
• one is minor structural
• final one is the crux of the PR

It'll probably be quicker to review each one individually!

@marbre I think this'll be an easy one! Could you review it?

[marbre]

Thanks for going over this. Some first comments.