Basic attempt to implement the README card functionality

[lackerman]

As per the discussion on #25
this is the work I did to enable this functionality with a custom
plugin, now within the context of the Gitlab plugin.

[satrox28]

Hi @lackerman thanks, will take a look. Can you sign your commit to pass DCO.

[FlxPeters]

Is there anything to do or help in order to get this PR merged?

[satrox28]

Hi @FlxPeters can you please take a look and see if the readme card renders the markdown correctly, last time when we checked it had issues in rendering the images from markdown.

[FlxPeters]

I will check it and may be add a new PR. My use case is to have an brief overview and long description for components.
So images do not really matter for me, but i understand it's not nice when they are broken.

[s-diez]

Hi I just tried the plugin and noticed some issues.

• Shouldn't the EntityGitlabReadmeCard be exported in the index.ts
• The default branch currently set to master should be configurable through an annotation. Main is quite common now.
• Should the card be displayed on the gitlab tab (EntityGitlabContent)

About the images I tested the following. I think the result is as expected.

• Images with full URLs work
• References to local files (relative URL) do not work.