[Merged by Bors] - Add a dedicated icon for a main branch docs site

[james7132]

This adds a PNG icon for denoting the development docs made from bevy's main branch. Added as suggested in bevyengine/bevy#3535.

If it doesn't show up in the diff, this is how it looks right now:

[cart]

Ultimately I'd like to make a number of changes to this:

• A vector source file (svg) instead of raster (png) to avoid scaling artifacts (the bevy birds have clear scaling artifacts) and get cleaner edges (the border between the hats and the birds is a bit uneven at times)
• Slightly less saturated colors for the text and hats
• Try the Fira Sans font for the text (which we use for a lot of Bevy content)

I think I'll hold off on merging this until we've iterated a bit, given that we haven't yet published a git version of the docs.

[james7132]

Unfortunately, I was not the one who made this (family member of mine did) and there is only a raster available.

Here's the original PSD file. The text was done in Adobe Illustrator, but was rasterized upon export to Photoshop.

[cart]

Closing for now because I'd like something that stylistically matches the base logo (ideally in svg form).

[aevyrie]

(font might not render properly on web)
Updated to use the simpleicons logo, aligned the text:

[alice-i-cecile]

The fresh icon is nice! A main branch docs site will be very useful, both for development and for testing of new book content per #177.

[cart]

bors r+

[bors]

Pull request successfully merged into master.

Build succeeded:

• build