fix links in entry HTML files other than index.html

Fixed internal links in entry HTML files other than index.html.

Removed links to markdown files that otherwise used to
generate HTML files other than index.html.

Ran `make gen_other_html` to perform the above.

2013/hou/doc/example.html

@@ -261,7 +261,7 @@ indirect source.</p>
 of up to 32 volumes, each with 64x64x64 cells. The volume 0 is the ‘top level’
 volume where the camera goes. Each cell of a volume can be empty, filled with a
 material, or another volume. The roundish characters in
-<a href="../old_default.scene">old_default.scene</a> and the carpet on the floor in this
+<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2013/hou/old_default.scene">old_default.scene</a> and the carpet on the floor in this
 file are examples where a cell is defined to another volume. Materials will be
 explained later.</p>
 <p>The volume shader for the i-th volume is named as the (i+1)-th capital letter.