meson: Generate documentation

[Pamplemousse]

I tried to convert the doc/local.mk as closely as possible;

• As some recipes were borderline shell scripts, and with meson's command feature not being very flexible, I ended up writing scripts in scripts/manual to be able to perform the needed actions.
  Also, I noted that some scripts were very similar, but I didn't try really hard to turn that into a unique parameterized script (it's not trivial);
• It relies on doc/manual: don't need to copy highlight.js manually NixOS/nix#5094 but fix could always be added if the PR is not accepted upstream (merged!);
• meson install failed because of compilation errors, so I couldn't check that.

cc @Ericson2314

[p01arst0rm]

looks good, nice work! it may be easier to seperate out the generation of different files into different directories, like is done in the include & dependency directories. makes things a bit easier to find :)

[p01arst0rm]

could you please newline these file names to improve readability?

[Pamplemousse]

Although I generally agree, I don't for that file:
There is a substantial amount of source files that are declared, forcing to scroll quite a bit already to go to the interesting part (IMO, executable declarations are more important than lines and lines of files).

[p01arst0rm]

@Pamplemousse i'm pretty sure the command to install needs to be sudo :)

[p01arst0rm]

@Pamplemousse @Ericson2314 as NixOS#5094 has been merged, ill move the meson branch to the latest master then fix the build. should then be able to merge this too (provided no conflicts).

[Ericson2314]

@p01arst0rm I would merge this then rebase, now that all comments are addressed.