…time

I'd like to make this distinction even clearer in the future, but it'll
have to wait for another doc push.  For now, at least provide a pointer
to anyone coming to this page wanting to update the runtime.

Trying to provide a more useful resource to point people to than a
smattering of scattered doc snippets.

There's still some places this could be better cross-referenced (e.g. in
--help output) and more that could be said about runtimes in general and
each runtime specifically… but that can all come in due time.  I needed
to find a stopping point for this work _somewhere_.   A big gain here is
that now there's clear places to attach additional documentation in the
future.

Like the recent switch from dynamically- to statically-generated rST for
command documentation, it may also sooner-than-later make sense to ditch
the "automodule" directive and do something static instead.

Resolves <#288>.

Previously undocumented!

Since we immediately convert the Docker/OCI image to SIF, the Docker/OCI
image layer caches end up being duplicative.  Not keeping them cached
means the cache can't be used to avoid downloading layers that haven't
changed in subsequent updates, but I think when choosing between
minimizing transfer or storage for our users, we should minimize
storage.

If we want to have our cake and eat it to (i.e. minimize both), we might
be able to change tack to newer Singularity features where (I think) OCI
images can be used directly without first converting to SIF.  Then, like
with our Docker runtime, the image we use and layer caches are one and
the same.  That's a larger change for a future time, though.

Resolves <#225>.

…ary with ours

More internally consistent now that we have the glossary term, which in
turn links to our overview page about them for more details.

…ng/updating runtimes

A good suggestion by @joverlee521 in review.

à la how we do it for the docker-base repo and the Docker runtime.

A good suggestion by @joverlee521 in review.

…f our ambient runtime

The general installation page contains a good example of ambient setup.

A good suggestion by @victorlin in review.

…ean up