markdown/yaml files for project description and landing pages

Projects should be allowed to publish files to show own landing pages in tgrep. Like explanation text about the projects goal or what is published within.

One idea is to publish a file index.md (markdown) which contains text which is shown on a project specific page. This file could in a later stage also contain a yaml header, allowing more project speficic settings of how to present the project in tgrep-portal.

Especially useful would be yaml config for project description and project title (as these are not changeable in tglab).

The mimetype shall be text/markdown -> https://tools.ietf.org/html/rfc7763