docs(developer-guide): include developer guide in compodoc generated docs

[christophercr]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalBankBelgium/ngx-form-errors/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[X] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The Developer Guide is not included in the generated docs by compodoc. Ideally, this Dev Guide should not only be reachable via Github but also via the published docs website (when that is available).

What is the new behavior?

• refactor compodoc configuration in a .compodocrc.json file to make it more readable and flexible in order to include the different MD files under /docs to create the Developer Guide.
• fix not well-formed links and adapt some urls in the Developer Guide

Does this PR introduce a breaking change?

[ ] Yes
[X] No

[SuperITMan]

😊

[SuperITMan]

😊