docs: Add info on chipyard's tests directory and how to do multithreading in RTL simulations

[JL102]

The most important part of this addition, IMO, is the note about __main(), since it's currently not noted anywhere in Chipyard's docs. It was initially very hard for me to find any explanation about __main until I stumbled across libgloss-htif's readme page.

@ maintainers: I don't actually know for sure if libgloss-htif is the right place to link to. It's the only documentation I could find that includes an explanation on __main, but I don't know if that's actually the code used for Verilator/VCS simulations.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• [N/A] Did you delete any extraneous prints/debugging code?
• [N/A, no permissions] Did you mark the PR with a changelog: label?
• [N/A] (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• [N/A] (If applicable) Did you add a test demonstrating the PR?

• [N/A] (If applicable) Did you mark the PR as Please Backport?

[jerryz123]

Thanks for adding this. I suggested some changes to make this even more clear.