Make MD conversion service pluggable

a.dmitriiev → made their first commit to this issue’s fork.

As this issue ✨ Add an admin page for module parameters Active was done without taking into account the requirement from this issue, I have implemented the admin UI in MR from this issue with a new properties and new schema.

The MR provides html to markdown converter plugins and plugin manager for it. It is possible to change the default converter in admin UI and also the settings for all converters.

One small issue:
'#description' => $this->t("Set to true to turn 'br' into '\n' instead of ' \n'"),
in src/Plugin/HtmlToMarkdownConverter/LeagueHtmlToMarkdown.php is run through an HTML filter and the \n are suppressed.
I ended up with this, which looks a little better and is more explicit:
'#description' => $this->t("Set to true to turn 'br' into a simple newline '\\n' instead of two spaces followed by newline '  \\n'"),

Also added our new co-maintainers to README.md while I was at it.

The PR works well in my hands and code looks really clean, setting issue to RTBC.

All the comments were addressed. Please review.

Merged into 1.0.x branch.

Released with v1.0.0