[Suggestion] Use a Commonmark parser for the rendering of readmes

Currently, the rendering of readmes has 2 different paths:

- using the Github rendered readme for github repos, which ensures that we get the same parsing than on Github (as Github is doing it)
- rendering the markdown with `cebe/markdown` for other packages (as we don't have pre-rendered readme for them)

This suggestion is about replacing `cebe/markdown` for the second case.

Nowadays, most non-github repositories on packagist are probably Gitlab ones. And Gitlab's markdown is based on CommonMark (with a few extensions of their similar to the Github ones, see https://docs.gitlab.com/ee/user/markdown.html). And Bitbucket also uses CommonMark (https://confluence.atlassian.com/bitbucketserver/markdown-syntax-guide-776639995.html).
So using a CommonMark parser would probably be better for compatibility of the rendering. But `cebe/markdown` is not such a parser.