feat: init wdl-doc

[a-frantz]

Describe the problem or feature in addition to a link to the issues.

This is a very rough init commit for wdl-doc. All the current HTML output is going to be thrown out and reimplemented once we have the templates from the designers, so I wouldn't spend much time combing through that with a fine-tooth comb.

The most important bit included in this PR is the resulting output directory structure created in wdl-doc/src/lib.rs.

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[peterhuene]

Looks great, just some nit picky comments.

[claymcleod]

Just a thought for the future—it may be worth pulling in the St. Jude Cloud workflows as a submodule here for a more complete picture.

[claymcleod]

Good after these.