Add MkDocs format and differentiate repository format from markdown format

[gallyamb]

Hello!

This PR adds support for MkDocs markdown format

Also it adds new command option to specify the repository management system type (the option is named repository.type, but it's discussable). New option is required to differentiate the format of the Markdown rendering system from the format of the Repository management system, because they are not dependent from each other

Will be glad to hear your feedback!

[gallyamb]

@princjef hi! Would you mind to look at this PR? Any feedback will be appreciated!

Also, there are pending test actions waiting for your approval, as I understand. Could you start them, please?

P.S.: tagged you because it seems like you missed notification about this PR. Sorry if I'm wrong