README updates

[jordancarlin]

• Remove list of ISAs that have Sail implementations. This is already in the Sail repo README and doesn't contribute anything to the RISC-V specific README
• Refer to the Extension Roadmap Wiki page for the list of features that is not yet supported. I've spent a bunch of time lately going through this Wiki and updating it with a list of (as far as I can tell) just about every ratified extension that we don't yet support. The only extensions I didn't bother to include are many of the profile defined extensions that are really just architectural options.
• Update sample Sail code to match the current implementation of the ITYPE instructions.

The other thing that is currently out of date with the README is the directory structure. I could update it, but I'd be inclined to just remove it because I don't think it provides much value and it will continue to fall out of date.

[github-actions]

Test Results

396 tests  ±0   396 ✅ ±0   1m 19s ⏱️ -1s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit ef35a4e. ± Comparison against base commit 91da83e.

♻️ This comment has been updated with latest results.

[Timmmm]

LGTM, and I agree we can remove the directory structure.

[jordancarlin]

Removed directory structure

[pmundkur]

I'm not sure removing the directory structure is a good idea. It's a useful orientation and pretty easy to keep up-to-date, unlike the rest of the documentation. The other changes LGTM.

[jordancarlin]

Added directory structure back and updated it to match the current state of the repo