CI: Build HTML documentation.

[alexvong243f]

Closes #1157.

• .github/workflows/main.yml: Add step in 'bist' job accordingly.

[cbm755]

This adds 4 mins to each BIST job. Maybe we should just do it once, say in that generic Ubuntu job...? Or as a brand new job.

If there is an error in the docs, does this fail?

[cbm755]

I tried deleting a tag and it certainly does die:

octave-help-XYWRQY:145: `@end' expected `example', but saw `defmethod'
octave-help-XYWRQY:145: `@end' expected `example', but saw `defmethod'
error: __makeinfo__ returned with error code 1
. Couldn't parse      texinfo:
###### OCTAVE START ######

@documentencoding UTF-8
@defmethod @@sym atan (@var{x})
Symbolic atan function.

Example:
@example
@group
syms x
y = atan (x)
  @result{} y = (sym) atan(x)
@end group

Note
error: called from
    __texi2html__ at line 45 column 5
    __html_help_text__ at line 35 column 17
    generate_package_html>wrote_html at line 842 column 5
    generate_package_html at line 207 column 7
make: *** [Makefile:79: tmp/symbolic-html] Error 1

So this is good. Just not completely sure we should do it every job, but that could be tweaked later.

[alexvong243f]

Agree, we only have to build the HTML doc in one job, as the HTML doc doesn't change with the octave and sympy version.