doc: adjust txt2tags usage for versions >=3.3

doc: adjust txt2tags usage for versions >=3.3

1) drop --css-sugar option from txt2tags as it is no longer supported

Quoting from https://sources.debian.org/src/txt2tags/3.4-2/CHANGELOG.md/,
respectively /usr/share/doc/txt2tags/changelog.gz as present in current
Debian/unstable:

| * Always use CSS sugar and drop `--css-sugar` option (Jendrik Seipp).

2) the xhtml target is also gone since v3.3

Quoting the changelog:

| # Version 3.3 (2019-11-04)
|
| * Update html target to HTML5 and drop xhtml target (Jendrik Seipp).

Also see https://github.com/txt2tags/txt2tags regarding v2.7 vs. v3.x

NOTE: the output of txt2tags v2.6-4.1 (as present in Debian/buster) is
not really useable now. We don't depend on txt2tags >=3.4-2 as present
in Debian/unstable though, as we don't use the HTML build output during
the package build.