Strip comments in theme/sekeleton CHANGELOG.md when displaying in grav website

[eauchat]

Seem like html comments in themes and skeletons CHANGELOG.md files are displayed in the https://getgrav.org/downloads/themes page. I guess that page simply parses the markdown file with a markdown parser. If the parser used supports stripping html comments, maybe it'd make sense to do so.

To see the problem in action, you can compare the beginning of those various views of the same file:
Compare the changelog displayed in getgrav.org, to the way github for example parses the markdown file.
Here is the actual content of the file.

The initial section could be removed in order to avoid the not so pretty display in getgrav.org. But it's also convenient to have the changelog file format described at it's beginning, so that when one edits it, they have the format documentation directly available.