Website update, fix for bug 13933, and list libc-testresults ML.

[Carlos O'Donell]

I have bundled a few things together in the website update to
get feedback from the community.

Updated website:
https://www.gnu.org/software/libc/preview/libc.html

(a) Restructured the website to provide more direct answers to:

    * What is glibc?
    * How do I get started?
    * How do I get involved?

    Also provide direct no-nonsense developer links to:

    * Sources
    * Documentation

    This fixes bug 13933 "web site links to git are not so easy to find"
    by adding a sources tab.

(b) Add information about the new mailing list libc-testresults:

    On the wiki:
    https://sourceware.org/glibc/wiki/MailingLists

    On the website:
    https://www.gnu.org/software/libc/preview/libc.html (News entry)
    https://www.gnu.org/software/libc/preview/involved.html (Reference)

Comments appreciated.

-- 
Cheers,
Carlos.