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/netslide.html

@@ -0,0 +1,14 @@
+Netslide
+<p>
+Slide the grid squares around so that they all join up into a single
+connected network with no loops.
+<p>
+Click on the arrows at the edges of the grid to move a row or column
+left, right, up or down. The square that falls off the end of the
+row comes back on the other end.
+<p>
+Squares connected to the middle square are lit up. Aim to light up
+every square in the grid (not just the endpoint blobs).
+<p>
+Connecting across a red barrier line is forbidden. On harder levels,
+there are fewer barriers, which makes it harder rather than easier!