Hi! This pull request updates the Ruby extension documentation for [the
upcoming v0.9.0
upgrade](https://github.com/zed-extensions/ruby/pull/106):

- Added documentation for two newly added language servers: `sorbet` and
`steep`.
- Updated documentation on using the `ZED_CUSTOM_RUBY_TEST_NAME` symbol
for tasks.

Thanks!

Release Notes:

- N/A