Update docs/README.md

[aishasalim]

The instructions for building the documentation no longer worked:

• The build command was given as docs/mdoc via SBT, but the project moved to Mill; the command is now ./mill mdoc.
• Building requires Scala CLI and Verilator on the PATH, neither of which was mentioned. Without them the build fails partway through with Cannot run program "scala-cli" or verilator not found on the PATH!, which is not obviously a missing-dependency problem.
• The website was described as a separate repository with edits picked up from the 3.4.x backport branch. It now lives in website/ in this repository and is built with Docusaurus, so the sidebar link pointed at a file that no longer exists.
• The mdoc:verilog example called ChiselStage.emitVerilog, which has been replaced by emitSystemVerilog.
• The links to the per-directory "Contents" pages pointed one directory too deep (src/cookbooks/cookbooks.md rather than src/cookbooks.md).

Adds a section listing where each kind of documentation belongs, and one covering how to render the website locally, both of which were asked for in the issue.

Fixes #4402

Summary

Release Notes