Keep thread pools documentation separate

[pitr-ch]

followup of #213
cc @jdantonio @jrochkind

[coveralls]

Coverage remained the same when pulling 80c5495 on pitr-ch:master into a7acb85 on ruby-concurrency:master.

[pitr-ch]

@jrochkind moving the thread_pools.md file from the inclusion to being stand-alone. I agree it makes more sense.

@jrochkind I agree it would be nice to make links also work on github. It would have to be easy to maintain though.

Which brings me to the question of separated .md files in doc directory. As Jerry mentioned they are easier to edit and maintain (I agree) so It's better to keep the files there, we are already struggling with documentation completeness and up-to-dateness so I think we should also be thinking about making it easy for us.