Add previous button

[patricklaf]

I like the next button defined in custom.css but I needed previous button.

This PR add previous button classes and also a section_buttons class to arrange buttons nicely when you have both previous and next buttons.

Setup

Add custom.css to the HTML_EXTRA_STYLESHEET of the Doxygen configuration file:

HTML_EXTRA_STYLESHEET  = doxygen-awesome.css custom.css

How to use?

Previous or next button

Add a span with the right class, previous_section_button or next_section_button:

<span class="previous_section_button">
Read Previous: [Firmware](firmware.md)
</span>

Previous and next buttons

If you want to have both buttons, just add an enclosing div with section_buttons class:

<div class="section_buttons">
<span class="previous_section_button">
Read Previous: [Hardware](hardware.md)
</span>
<span class="next_section_button">
Read Next: [Settings](settings.md)
</span>
</div>

[jothepro]

Nice, thank you! I think I'll also add previous buttons to the demo. Pls be patient with this MR, I will come back to you asap!