Add guidelines for deprecating tools. Deprecate jshint and jslint

[mre]

This fixes #223.
As discussed in the thread, I'm also adding guidelines on how to mark tools as deprecated/unmaintained to the documentation.

[impredicative]

Looks good. To be clear, there are certain criteria for which a tool would need to get unlisted, and the warning symbol wouldn't apply to those.

[mre]

To be clear, there are certain criteria for which a tool would need to get unlisted, and the warning symbol wouldn't apply to those.

Fully agree. For example, if critical security vulnerabilities don't get fixed in a timely manner. Maybe we should add a section about fully unlisting tools, but that's a bit out of the scope of this PR. Important point however.