Cleanup cpp_linter source code starting from `v2`

[shenxianpeng]

Since we have already abstracted python sources from cpp-linter-action to cpp-linter repo, and we'll change action from using docker to composite steps, it's ready to clean up cpp_linter related code starting from v2 release.

[2bndy5]

The docs should be altered (or removed), so that the only the README (& possibly the new contributing guidelines) are rendered. The API reference pages will no longer be needed if the python source code is removed. In fact, removing the python sources without altering the docs may actually break docs' build.

[2bndy5]

The dev-test.yml CI and the coverage badge should go too.

[shenxianpeng]

If #83 is ready to merge, it should be simple to merge it and then clean up the source code to avoid conflicts.

[2bndy5]

Submitting a review shortly