Add some jpm declare- docs (#34, #209)

[sogaiu]

This PR is an attempt to address #34 and #209.

It adds a new section to the jpm page that contains some docs for the following declare- callables:

• declare-bin
• declare-binscript
• declare-headers
• declare-manpage

I have seen each of these used in at least one project [1], so it seemed worth doing.

I based the added docs on the docstrings found in jpm's declare.janet. I made some changes though because some things didn't seem right or there seemed to be missing bits.

Also snuck in a minor formatting change to a code example. I hope that can be forgiven (^^;

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[1] I did not add a docstring for one other construct I found in jpm's declare.janet because I have not encountered a single project that uses it.

[pepe]

LGTM