Add logo to RTD and release # to TOC

- added favicon - file gitextensions/bin/logo/git-extensions-logo-final-32.png converted to .ico
- added logo - file gitextensions/bin/logo/git-extensions-logo-final-128.png
- add release number to title on TOC page

Read the Docs can display a logo on the HTML page sidebar and favicon on the browser tab. I have used the above two files from the main code repo for these images.

Note that the best size logo for RTD seems to be 200x200 pixels. I tried the final-256 PNG file from bin/logo but this was too large for RTD (too wide for the sidebar). Since there is no 200x200px file, I have used the 128x128px one, which looks ok.

(I know I didn't need a PR for this, but it just seems more visible to have them :)