Switch to sphinx doc

[tfcollins]

Move doc to sphinx

This PR switches doc to the new Sphinx flow followed by HDL, pyadi-iio, No-OS, and TransceiverToolbox. This includes:

• API doc for C through Breathe and Python (sphinx-autodoc)
• A number of wiki pages have been merged in on libiio's internals
• Original technical doc used within the Doxygen builds
• Build instructions (Windows, Linux, macOS)

There is still doc missing for:

• C# bindings
• Links to past revs
• CI to deploy generated pages

PR Type

• New feature (a change that adds new functionality)

PR Checklist

• I have conducted a self-review of my own code changes
• I have commented new code, particulary complex or unclear areas
• I have checked that I did not intoduced new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[gastmaier]

There is still doc missing for: Links to past revs

My idea is to track versions on gh-pages with a simple tags.json,
and implement the version dropdown selector on the sphinx theme, fetching this file from {global_root}/tags.json.
So I suggest creating one like this

[
  "v0.25",
  "v0.26",
  "main"
]

and keep the current landing page? or put main on root / ... for doctools I did that by creating an array with all doc versions, then git rm all, and finally restoring by git checkout every dirname from the array.

CI to deploy generated pages

https://analogdevicesinc.github.io/doctools/ci.html#rolling-release
https://analogdevicesinc.github.io/doctools/ci.html#versioned
with ADOC_TARGET_DEPTH=1 considering store paths like "main" and "v0.26"

[tfcollins]

ok I get the idea. The libiio doc isn't greenfield so I'll have to think about handling the older doc.

[rgetz]

Also missing cpp bindings doc.