Introduce a mechanism in this source tree for building the container

web pages for the Java applets. Previously, those have all been
maintained by hand in my website's svn area, which is a bit silly. Now
we have a file per puzzle in the 'html' subdirectory which contains
the puzzle's name, one or two attributes, and the instructions snippet
to go below the puzzle applet; and then there's a Perl script that
builds all the real web pages out of that by adding in the parts
common across all files: the header, footer, and middle fragment with
the <applet> tag and resizing bits and pieces.

One piece _not_ checked in here is the footer text specific to my
hosting at chiark, which I think does still belong in the www area. So
Buildscr doesn't actually build the web pages; it just delivers the
bits and pieces by which my nightly snapshot script will be able to
run the program that _does_ build them, passing that footer as an
extra argument.

[originally from svn r9780]

html/magnets.html

@@ -0,0 +1,14 @@
+Magnets
+<p>
+Fill each domino shape with either a magnet (consisting of a + and
+&#8722; pole) or a neutral domino (green).
+<p>
+The number of + poles that in each row and column must match the
+numbers along the top and left; the number of &#8722; poles must
+match the numbers along the bottom and right. Two + poles may not be
+orthogonally adjacent to each other, and similarly two &#8722; poles.
+<p>
+Left-click a domino to toggle it between being empty and being a
+magnet (the + is placed in the end you click). Right-click to toggle
+between empty, neutral, and a ?? mark indicating that you're sure
+it's a magnet but don't yet know which way round it goes.