Add documentation link into error reporting

[macisamuele]

The goal of this PR is to make explicit the link to the documentation page where the reporting is detailed explained.

[codecov[bot]]

Codecov Report

Merging #9 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #9   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          59     59           
  Lines        1780   1808   +28     
  Branches      113    115    +2     
=====================================
+ Hits         1780   1808   +28

Impacted Files	Coverage Δ
tests/cli/run_test.py	100% <100%> (ø)	:arrow_up:
tests/conftest.py	100% <100%> (ø)	:arrow_up:
swagger_spec_compatibility/rules/common.py	100% <100%> (ø)	:arrow_up:
tests/rules/common_test.py	100% <100%> (ø)	:arrow_up:

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[macisamuele]

@sjaensch this is now ready for review. As we discussed offline there was a "minor" issue related to the fact that the previous iteration would have provided read-the-docs links for all the rules (even if not implemented in the library) and this would eventually increase confusion)

The new approach does provide:

• an attribute to define the documentation link
• a fallback, for rules that are implemented in the lib, to generate the read-the-docs link if documentation_link is not defined