[search]: config content type that should be indexed

[thmarx]

We do not want some pages to be index (startpage or impressum). We need some kind way to stop the search module to index these pages.
Maybe a header field that is passed to the html.
https://developers.google.com/search/docs/crawling-indexing/block-indexing

this has effects to google to, so we have to think about that.

[thmarx]

We should have access to the metadata in the modules.

[thmarx]

We do not have content types yet. We could use the template name

[thmarx]

meta data:

search:
    index: false

do disable the search index.