[xtask] Add aggro subcommand for building aggregate docs

[jamesmunns]

I've currently plumbed a good "base coat" of the aggro subcommand (name bikeshedding welcome, it was funny on a Friday morning), which generally:

• Takes a task config
• Figures out all the tasks in that config
• Splats all of the READMEs (that are present) into a single bundle, and renders them into a single HTML "report".

Currently, it looks like this:

aggro.html

From now, there are a couple of questions:

1. I could use some feedback on "matters of taste":

• What information WOULD be useful to document on a per-app, or a per-task basis?
• I included some simple tables, but these should be considered a proof of concept.
• How do we want to shape this? One big MD/HTML/PDF file? Something more multi-page organized like mdbook?

2. I think we'll want to make a pass over our task docs:

• Most tasks don't have any docs! But we can backfill this :)
• We probably want to set out a bit of a "standard" of how drivers+tasks+apps are documented

I did try figuring out how to insert the task graph, sadly the launch-rs crate isn't as good at layouting as the dot bin is, and neither are great for making legible diagrams. We may want to break this up on a per-task basis just to reduce the number of entities per-graph.