Use Doctools

[gastmaier]

Changed to be merged to sphinx-web-documentation

[CLAassistant]

All committers have signed the CLA.

[gastmaier]

Also note that the Documentation/Update gh pages CI is running for PRs, I recommend removing on: pull_request from the ci file. Or having different targets (e.g, on pull_request build the doc but do not push to gh_pages branch)

[gastmaier]

1. identifying_virtual_serial_port.png doesn't exist in the repo and the problem existed before this commit; at d10867e7ab1da2fc8eca00689aeeda90caf867ba doc:Adding doc for ad777x_iio and nanodac_console:

   /mnt/wsl/data/repos/precision-converters-firmware/doc/sphinx/source/hardware/comm_interface.rst:: WARNING: image file not readable: source/hardware/identifying_virtual_serial_port.png

2. Same as above

3. There is an issue with the nesting/includes across the doc: The part pages start with:

   ADXXXX IIO Application
   ------------------------

   Then the sections header are:

   ==========
   Placeholder Text
   ==========

   And finally the included page starts with

   
   =======
   Build Guide
   =======

Build Prerequisites

That means, `ADXXXX IIO Application` and `Build Prerequisites` are in the same header level ([H1](https://www.w3schools.com/tags/tag_hn.asp)), causing the odd behavior. 

Sphinx sets the header level by first appearance, for example, if `~~~~~~~` appears first, it takes the H1 position.

A quick fix is to change the *_iio.rst header to:

ADXXXX IIO Application

Setting `==` as the H1 level, still not perfect, since the other sections should be `---` instead of `===` to take the H2 level.
Other solution is to change to:

ADXXXX IIO Application """"""""""""""""""""""""""""""

Now, the `"""` takes H1 position, `===` H2, `---` H3, and so on, and searching recursively, `"""` is never used in this doc.

doc/sphinx$ grep -rn '"""' .