Piccolo-theme

Added support for Python 3.6, as many Ubuntu systems will still be using that version, and Sphinx still supports it. Thanks to oncilla for reporting this issue.

Improved the appearance of autodoc output for C files (when using [breathe](https://breathe.readthedocs.io/en/latest/)). Courtesy thijsmie.

Changed the arrow symbols - they didn't look great on mobile.

Added spacing between sections, so it's not necessary to add horizontal dividers any more.

rst
My Heading
=========

Section 1
---------

Some content

-------------------------------------------

Section 2
---------

Some content


We can now just do:

rst
My Heading
=========

Section 1
---------

Some content


Section 2
---------

Some content


Other minor changes:

* Using unicode triangle character instead of < for some links
* Plain admonitions are now styled properly:

rst
.. admonition:: A custom admonition

This is my custom admonition!

Improvements to the notification feature - it was causing too many browser reflow operations.

A notification can now be shown at the top of each page:

<img width="823" alt="159574084-2079419a-303c-4e17-8181-0076d5ac64b5" src="https://user-images.githubusercontent.com/350976/159690539-53758410-5b04-4d16-a35a-657e47e0cc5a.png">

This is how:

python
conf.py
html_theme_options = {
"banner_text": 'Welcome to our amazing documentation!',
"banner_hiding": "permanent"
}


This involved quite a few CSS changes - please clear your browser cache if anything appears broken.