Replace trac links

[mkoeppe]

Note: After merging, we can remove https://github.com/sagemath/sage/wiki/WikiStart

[haraldschilly]

Note: there was a conflict on the download page. The section after my manual merge looks like that:

  <p>
    <strong>Note:</strong> <strong>Linux</strong> binaries have been discontinued. See the <a href=
    "https://doc.sagemath.org/html/en/installation/index.html">
    Installation guide</a> for alternatives.
  </p>

If this is wrong, please do a follow up PR.

[haraldschilly]

... and something is off with a template. I'll look into this.