improve the footer of HTML files

We improve the `inc/footer.default.html` file to make
the _Copyright and CC BY-SA 4.0 License_ clear.

We add `license.md` and generate `license.html` (via `make
gen_top_html`) so that key features and terms of the _Copyright and
CC BY-SA 4.0 License_ are accessible.

Add CC BY-SA 4.0 license images into the `png/` directory.

FYI: We use the license to allow the open use of IOCCC entry code,
and at the same time, we help ensure that those working on the IOCCC
have proper Attribution including, of course, the IOCCC winners
themselves!  The _Copyright and CC BY-SA 4.0 License_ help ensure
that everyone may enjoy the IOCCC.

Added '#copyright' to all generated HTML pages so that the
_Copyright and CC BY-SA 4.0 License_ may be reached.

Changed `bin/gen-top-html.sh` VERSION from "1.2.2 2024-03-26"
to "1.2.3 2024-04-12".

Fix `bin/gen-sitemap.sh` to remove requirement of `ioccc.tar.bz2`.

Changed `bin/gen-sitemap.sh` VERSION from "1.3.4 2024-03-27"
to "1.3.5 2024-04-13".

Perform `make www sitemap` to perform the above.

We also give a _hint_ as to the next project.

TODO: Complete that next project we are _hinting_ at.

TODO: Review the work being done by a web designer to see
      how their work might be incorporated into the web site.

bin/gen-top-html.sh

@@ -88,7 +88,7 @@ TOP_MD_SET+=("faq")
 
 # set variables referenced in the usage message
 #
-export VERSION="1.2.2 2024-03-26"
+export VERSION="1.2.3 2024-04-12"
 NAME=$(basename "$0")
 export NAME
 export V_FLAG=0
@@ -123,6 +123,7 @@ TOP_MD_SET+=("faq")
 TOP_MD_SET+=("inc/README")
 TOP_MD_SET+=("index")
 TOP_MD_SET+=("judges")
+TOP_MD_SET+=("license")
 TOP_MD_SET+=("news")
 TOP_MD_SET+=("thanks-for-help")
 #