ENT-14135: Added broken internal link checker

[aleksandrychev]

Run it on the postprocess stage when we have built HTML files.

Ticket: ENT-14135

[aleksandrychev]

@cf-bottom jenkins please

[cf-bottom]

Sure, I triggered a build:

Jenkins: https://ci.cfengine.com/job/fast-build-and-deploy-docs-master/150/

Documentation: http://buildcache.cfengine.com/packages/build-documentation-pr/jenkins-fast-build-and-deploy-docs-master-150/output/_site/

[aleksandrychev]

nice, it works:

  ERROR: 7 broken internal link(s)/asset reference(s) found in the built site:
  reference/masterfiles-policy-framework/index.html:357: ../reference/language-concepts/policy-evaluation/
  reference/masterfiles-policy-framework/index.html:925: ../reference/language-concepts/policy-evaluation/
  reference/masterfiles-policy-framework/index.html:947: ../reference/language-concepts/policy-evaluation/
  examples/tutorials/policy-writing/policy-writing-with-cfbs/index.html:408: ../hugo-commit.png
  examples/tutorials/file_comparison/index.html:180: file_compare_test.cf
  examples/tutorials/promise-type-module-development/index.html:318: .././reference-promise-types-custom.html
  examples/tutorials/cfbs/index.html:168: ../../../getting-started/01-installation/```
  
  gonna fix errors

[aleksandrychev]

@cf-bottom jenkins please

[cf-bottom]

Alright, I triggered a build:

Jenkins: https://ci.cfengine.com/job/fast-build-and-deploy-docs-master/152/

Documentation: http://buildcache.cfengine.com/packages/build-documentation-pr/jenkins-fast-build-and-deploy-docs-master-152/output/_site/