Rename *.text *.md

As luck would have it the only entries where this applied were my own.
The links/references to these files have been changed in the files.

An unfortunate problem is that in 2020/ferguson1 there are several
*.markdown files. I will have to go through these and figure out which
ones should be changed to .md and which ones can be removed. This can
happen later but in the meantime the references are to .md. I believe
the only difference is that the .md has the footer added by the judges.
This was a workaround with the fact that I wasn't sure (and still am not
entirely sure) the generation of the html files will work. It might be
that the .markdown files have the footer and the .md files do not but
this can be dealt with later anyway.

2020/ferguson1/bugs.html

@@ -153,7 +153,7 @@ way negative size will force a win.</p>
 head several places until you end up getting to the max size (by way of it
 being unsigned). This isn&rsquo;t a bug even if it might seem like one. In fact
 this is the basis of the grow/shrink mode of play that I talk about in the
-<a href="gameplay.markdown">gameplay.markdown</a> (<a href="gameplay.html">gameplay.html</a>) file.</p>
+<a href="gameplay.md">gameplay.md</a> (<a href="gameplay.html">gameplay.html</a>) file.</p>
 
 <p>Here&rsquo;s another thing to be aware of. Let&rsquo;s say you do:</p>
 
@@ -270,7 +270,7 @@ that 49 is the max snake size. How is this derived and why?</p>
 (<code>playable_rows * playable_columns</code>). I have played with this many
 times and I do not believe there should be a problem. If there is it&rsquo;s from a
 very poor implementation of rand() and I discuss this in
-<a href="troubleshooting.markdown">troubleshooting.markdown</a> (<a href="troubleshooting.html">troubleshooting.html</a>).</p>
+<a href="troubleshooting.md">troubleshooting.md</a> (<a href="troubleshooting.html">troubleshooting.html</a>).</p>
 
 <p>There I also give some test outputs without the cap in place to give an idea of
 when it became a problem (or would have). In short this is known but I do not