fix: address bugs found during first run on Ruby

[bcoe]

1. we now check the last 100 PRs, when deciding whether to close stale PRs.
2. we now show a "Documentation" section in changelog: only with one entry for all docs, only if at least one docs change exists.
3. our logic for updating version.rb was slightly off (it needed to replace - with /).

[codecov]

Codecov Report

Merging #238 into master will increase coverage by 0.06%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   77.73%   77.79%   +0.06%     
==========================================
  Files          21       21              
  Lines        2565     2581      +16     
  Branches      192      193       +1     
==========================================
+ Hits         1994     2008      +14     
- Misses        559      561       +2     
  Partials       12       12

Impacted Files	Coverage Δ
src/github.ts	56.11% <100%> (ø)	⬆️
src/conventional-commits.ts	93.51% <88.23%> (-0.49%)	⬇️

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