docs: Replace sourceforge logo with a text link

docs: Replace sourceforge logo with a text link

Fixes the following Lintian (Debian package checker) error:

privacy-breach-logo

  usr/share/doc/mesa-common-dev/contents.html
    (http://sourceforge.net/sflogo.php?group_id=3&type=1)
  usr/share/doc/mesa-common-dev/thanks.html
    (http://sourceforge.net/sflogo.php?group_id=3&type=1)

The extended description of this tag is:

    This package creates a potential privacy breach by fetching a logo
at runtime.

    Before using a local copy you should check that the logo is suitable
for main. You can get help with determining this by posting a link to
the logo and a copy of, or a link to, the logo copyright and license
information to the debian-legal mailing list.

    Please replace any scripts, images, or other remote resources with
non-remote resources. It is preferable to replace them with text and
links but local copies of the remote resources are also acceptable as
long as they don't also make calls to remote services. Please ensure
that the remote resources are suitable for Debian main before making
local copies of them.

    Severity: serious, Certainty: possible

    Check: files, Type: binary, udeb

Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>