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/gameplay.html

@@ -87,11 +87,11 @@ for the input keys.</p>
 <h1>1. <a name="support" href="#toc">Testing terminal environment/capabilities</a></h1>
 
 <p>Most people probably won&rsquo;t have any issue at all but for those who have any
-problems with the terminal see the <a href="terminals.markdown">terminals.markdown</a> file for more
+problems with the terminal see the <a href="terminals.md">terminals.md</a> file for more
 information on the terminal capabilities testing utility I include.</p>
 
-<p>For troubleshooting other problems see <a href="troubleshooting.markdown">troubleshooting.markdown</a>
-(<a href="troubleshooting.html">troubleshooting.html</a>) and <a href="bugs.markdown">bugs.markdown</a> (<a href="bugs.html">bugs.html</a>).</p>
+<p>For troubleshooting other problems see <a href="troubleshooting.md">troubleshooting.md</a>
+(<a href="troubleshooting.html">troubleshooting.html</a>) and <a href="bugs.md">bugs.md</a> (<a href="bugs.html">bugs.html</a>).</p>
 
 <h1>2. <a name="input" href="#toc">Input keys</a></h1>
 
@@ -441,7 +441,7 @@ less than again - I&rsquo;m not going to try and figure that out however). Anywa
 </code></pre>
 
 <p>Observe that the size isn&rsquo;t the default 997: because the screen size is too
-small for that value (see <a href="terminals.markdown">terminals.markdown</a>).</p>
+small for that value (see <a href="terminals.md">terminals.md</a>).</p>
 
 <p>The reason I chose 997 is that it leaves a bit of a challenge for some (maybe
 most) people but it&rsquo;s still winnable. Previously even I couldn&rsquo;t have won unless
@@ -471,7 +471,7 @@ the game is won is 199 - also a prime!</p>
 
 <h1>4. <a name="modes" href="#toc">Gameplay modes</a></h1>
 
-<p>Here I list a number of different modes of play. In the <a href="bugs.markdown">bugs.markdown</a>
+<p>Here I list a number of different modes of play. In the <a href="bugs.md">bugs.md</a>
 (<a href="bugs.html">bugs.html</a>) file I even show how there&rsquo;s a built-in test unit for some
 modes! There are certainly other modes that could be imagined: the modes below
 happened over time and some were simply an accidental discovery or something I