Support running existing mkdocs plugins and themes

[oprypin]

• Support running existing mkdocs plugins by redirecting the import

  First of all, support 'mkdocs.plugins' entrypoints but with a lower priority than ours.

  At the start of execution, set up a metaimporter such that whenever a plugin imports anything from 'mkdocs.*', the import gets redirected to us.
  Also keep MkDocsConfig as an alias for the same reason - it was the only renamed class.

• Support loading existing mkdocs themes

  Simply support the mkdocs.themes entrypoints but give preference to our entrypoints.

  Also, populate the mkdocs_version field for themes but with our project name prefix, to distinguish them if the theme renders this value.

[Andre601]

Should we perhaps inform users via warnings about "legacy" projects being used if technically possible?

Like WARNING - Legacy MkDocs plugin {plugin} loaded! If possible, use a properdocs alternative or ask the dev to support ProperDocs.

[oprypin]

Yes I have something for that but I realized that it's conceptually worth being in a separate pull request.

[oprypin]

@Andre601 Oh maybe I misread your message.
I think it would not be good to write this nagging message for almost every plugin. We will also not be forcing plugins to stop supporting mkdocs, it's fine for them to keep support both.
I was thinking the nagging should focus only on the mkdocs binary itself, not on plugins that still work under mkdocs.