Improve the compatibility of the doc between Github and RTD...

[lrmoreno007]

Added the installation of the nbsphinx package that ensure the compatibility of the documentation between Github and RTD.

The admin in ReadTheDocs must ensure that:

• In advanced setting, set requirements file: doc/requirements.txt <---- Use this exactly
• In advanced setting, ensure Python interpreter are: CPython 2.x or CPython 3.x

Now we must always use GitHub links and now the links directly work in ReadtheDocs. Progressively we must change the links of the :doc: and :ref Sphinx native types that exist in the documentation (but first this PR must be approved).

Special thanks to: @mgeier

Read more... #4409

[lrmoreno007]

I changed all the links with Sphinx native style (:doc: and :ref:) to GitHub style.

[d-a-v]

Out of curiosity How are you viewing the result in readthedoc.io ?

In advanced setting, set requeriments file: doc/requeriments.txt <---- Use this exactly

Can you double-check the spelling ?

Note that I'm not questioning the validity of this PR.

[lrmoreno007]

Excuse me, must be doc/requirements.txt

I see the results in my repo that is a translation to Spanish of this one.
http://esp8266-arduino-spanish.readthedocs.io/es/latest/index.html

Example:
Click in Software Serial
GITHUB https://github.com/lrmoreno007/ESP8266-Arduino-Spanish/blob/master/doc/reference.rst#serial
READTHEDOCS https://esp8266-arduino-spanish.readthedocs.io/es/latest/reference.html#serial

[devyte]

@lrmoreno007 this sounds pretty cool. Let me know when you're done, and I'll merge.

[lrmoreno007]

It's ready to merge if you want.