Move documentation to esp-docs and docs.espressif.com (IDFGH-7119)

[igrr]

Currently component documentation is built with Sphinx and hosted on Github Pages.
For consistency with other Espressif projects (both visual style and location) we should try to use esp-docs for building the docs and docs.espressif.com for hosting.
esp-modbus project docs can be used as an example.

[boraozgen]

Related: The references to this repository from the main IDF documentation and the esp-idf examples are missing. There are entries for mDNS and ASIO in the docs, but e.g. esp-modem is completely missing. Also there are no references in the examples directory. I discovered this repo by chance after tracking down in git why the pppos examples are deleted in the latest version. Currently from the main esp-idf repo it seems that there is no cellular modem support in the framework.