The raw HTML was invalid, and this way we use the common website template
for a uniform look without any duplication.
It seems gf-refman.html was once generated from txt2tags, although I have
been unable to find this original .t2t file.
I also tried to re-generate txt2tags from HTML but was not able to.
However I was able to convert HTML to Markdown using Pandoc and I think
the result is pretty good, so I think we should use this.
The original gf-refman.html can be obtained from git history, e.g.:
a7e43d872f/doc/gf-refman.html
+ Rename some txt2tags file from .txt to .t2t and remove abandoned .txt files.
+ Add program update_html that finds all .t2t documents and updates the
corresponding .html file. It can be invoked with 'make html'.
+ Add style to some .html documents
