Update unicode.texi to have items for sigma and mu

[ahubers]

Motivation

I was trying to find the unicode shortcuts for these and saw they're missing from the unicode documentation page. I've updated the src for this doc and run ./compile-docs.sh.

Note that because I used GNU Makeinfo 6.5 instead of 6.1, this resulted in the rest of the .html and .info files to be superfluously updated (as the comment at the top of these files now says "Created by GNU Makeinfo 6.5"). So I didn't bother to commit the rest of the files that were updated. I also noticed that it changed the page's title. I'm not sure why that is. @amarmaduke if you want me to commit the rest of these files let me know and I'll update the PR.

Also apologies again if these unicode shortcuts were deliberately left out of documentation because they're as of yet experimental features. In which case I can close this PR.

On Version 1.1.2

Let me know if mu and sigma notation exist on the latest released version of Cedille. If so, I can cherry-pick these changes onto https://github.com/cedille/cedille.github.io accordingly.

[ahubers]

Ah also ➾, which is \-, is missing. I can add this too...