Fill in gemspec description and summary

MSP-9795
Move documentation gems out of gemspec

MSP-9795

The documentation gems: yard, kramdown, and redcarpet, do not need to be
gemspec development dependencies because gems that depend on
metasploit-concern are not required to use its yard.rake.
Add semantic version pinning for activesupport

MSP-9795

`gem build` now warning if a gem isn't pinned using `~>` under the
assumption that most gems have adopted semantic versioning, so add it
for activesupport.