docs: major revamp of the Wrap format description [skip ci]

The current state of this manual can best be described as... confusing.
The flow of the page jumps from one topic to the next without ever
actually telling you what you can do, so it's almost impossible to keep
track of what is supported, while instead going into involved derails
about why you'd want to use a wrap, and scattering some (but not all)
information throughout the promo material.

The most important changes this rewrite does (aside from turning
supported keys into a list of bullet points) is adding documentation for
the lead_directory_missing property, and mentioning that wrap-hg and
wrap-svn exist. I had to find out all of this by reading the source code
implementation, so let's try to save other people the effort.

Other miscellania: as per @jpakkane's comment, take the opportunity to
point out that wrap dependencies are also useful on Linux, in cases
where your distro doesn't have a new enough version of "$dependency".
It's a fairly common problem outside of select rolling-release distros,
so well worth mentioning.

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