rename bin/gen-top-html.sh, added archive/historic/index.html

We rename `bin/gen-top-md2html.sh` to `bin/gen-top-html.sh`.

Modified the `bin/gen-top-html.sh` tool to build `archive/historic/index.html`
from `archive/historic/README.md`.

In `bin/gen-top-html.sh` changed VERSION from "1.1 2024-02-28"
to "1.2 2024-02-29".

Renamed `make gen_top_md2html` to `gen_top_html` in the top level `Makefile`.

Improved `archive/historic/README.md`.

Updated `robots.txt` to include the Sitemap directive.

Moved unused `valid-html401.png` to `png/valid-html401.png`.
This png image may be used later.

Performed `make gen_top_html` which rebuilt all top level HTML files
as well as `bin/index.html` and `inc/index.html`.

robots.txt

@@ -1 +1,3 @@
 User-Agent: *
+Allow: /
+Sitemap: https://www.ioccc.org/sitemap.xml