Recursively generate docs for all .nim files (excluding tests) in the prj dir

[kaushalmodi]

https://gitter.im/nim-lang/Nim?at=5bc9f064600c5f6423d4fee7

you can find the codegen for this: https://mratsim.github.io/Arraymancer/ in https://github.com/mratsim/Arraymancer/blob/master/nimdoc.cfg and https://github.com/mratsim/Arraymancer/blob/master/arraymancer.nimble#L185

[kaushalmodi]

Ref: https://github.com/jabbalaci/NimMultiModuleLibrary

---

Generate docs for a multi-modules library

This is an example that shows how to create docs for
a library that contains several modules.

Here we achieve the following things:

• We have a library that has a main module (src/stuff.nim)
  and some submodules (src/stuff/*.nim).
• We generate HTML docs for every *.nim file.
• We also generate an index file (theindex.html) that
  contains exported things from every module.
• The theindex.html file has links to every module.
  The links work :)
• If you click on the name of a module in theindex.html,
  you get to the HTML documentation of that specific module.

As putting all these things together took me some time and I
didn't find a step-by-step documentation, I assembled
this sample project.

Under Linux, I had to execute the following commands:

$ nim doc --index:on -o:docs/htmldocs/stuff.html src/stuff.nim
$ nim doc --index:on -o:docs/htmldocs/submodule1.html src/stuff/submodule1.nim
$ nim doc --index:on -o:docs/htmldocs/submodule2.html src/stuff/submodule2.nim

$ nim buildIndex docs/htmldocs/

$ mv docs/htmldocs/*.html docs/

# delete docs/htmldocs/

# open docs/theindex.html

See also 03_generate_docs.sh.

It would be nice if nim doc could do this with a switch. Generating
a complete documentation for a multi-module library should be easy.

Courtesy: @jabbalaci