Some markdown headers needed tweaking to adhere
to standards, including blank lines above and below,
only one space after hash, first header should be
h1, and only one h1 per doc

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues for many admin
docs (part 2)

- Also makes other minor Markdown fixes that were near the main fixes.

The Unicorn master resets any signal handlers and prevents rbtrace
from working properly as a result. Move the `require 'rbtrace'` call
to the Unicorn `after_fork` block and keep the initializer only for Sidekiq.

This will be enabled if ENABLE_RBTRACE environment variable is defined.
This will allow us to debug all sorts of issues in production:

https://github.com/tmm1/rbtrace

[ci skip]

iFoo

Correct the command to get a gdb backtrace from all threads. `apply` is
not a valid gdb command.

See https://sourceware.org/gdb/onlinedocs/gdb/Threads.html#Threads .

[ci skip]

[ci skip]