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/mines.html

@@ -0,0 +1,18 @@
+Mines
+<p>
+Try to expose every square in the grid that is not one of the hidden
+mines, without opening any square that is a mine.
+<p>
+Click in a square to open it. Every opened square are marked with
+the number of mines in the surrounding 8 squares, if there are any;
+if not, all the surrounding squares are automatically opened.
+<p>
+Right-click in a square to mark it with a flag if you think it is a
+mine. If a numbered square has exactly the right number of flags
+around it, you can click in it to open all the squares around it
+that are not flagged.
+<p>
+The first square you open is guaranteed to be safe, and (by default)
+you are guaranteed to be able to solve the whole grid by deduction
+rather than guesswork. (Deductions may require you to think about
+the total number of mines.)