Documentation in webpage

[eugenev]

Documentation restyle and re-organization

Previously the documentation of the various components were stored in each repository separately. In order to make these easier to maintain (and to make it easier to make cross-references, add a search function and check for dead links), I have migrated everything into a single repository: https://github.com/brocaar/chirpstack-docs. Apart from a restyle (using https://www.mkdocs.org/ and https://squidfunk.github.io/mkdocs-material/), I have re-structured a few pages to align with the MkDocs doc…