Make links more specific in gemspecs

[AlexWayfer]

• Finally, please add an entry to the corresponding changelog

I'm not sure should I add an entry to the changelogs, and each of them or just one.

Description

I believe each separate gem should have correct (direct, specific) links in its gemspec. In this architecture with single-repo it's achievable. Also it'll help us to list dependencies in our project without common duplicates to the root of the repo.

Additionally gems with specific version (such as 5.17.3) should point to documentation, changelog, sources, etc. of this version, not upstream (master).

Also links to rubydoc.info and GitHub issues were added.