132 add mqtt routing details

[vordimous]

• Update the general routing info for zilla
• Add a routing example to the mqtt broker guide
• general formatting

fixes #132 depends on https://github.com/aklivity/zilla-examples/pull/100

[vordimous]

Addressing the topic matching comments above, I went with the "reduce complexity and mention the exceptions" approach which seems to do the trick.