[Bug] libddoc should wraps standard sections with `DDOC_SECTIONNAME`

[Geod24]

Currently libddoc returns the content of sections "ad-hoc", while they should be surrounded with a macro.

We already have the macro definitions so it's just a question of mapping and doing the right thing when the section is standard, basically:

auto content = "$(" ~ SECTIONS_MACROS[toLower(sec.name)] ~ " " ~ sec.content ~ ")";
sec.content = expand(Lexer(highlight(content)), sMacros);

Here.

Depends on #24
Feel free to assign me.