Expose example filename creation via script

[sogaiu]

For background and motivation, please see this comment.

This PR is a suggestion to address #256 by:

1. Exposing the ability to generate a suitable examples directory filename for a Janet symbol via a script
2. Updating the "Adding Examples" section of the README to explain how to use the script

As part of point 1 above, a portion of content/api/gen-docs.janet was relocated to a new file named content/api/examples.janet. This latter file also contains a main entry point for use from the command line.

Use of the new script is also described in the udpated README, but a concrete invocation example is:

$ janet content/api/examples.janet array/new

which should yield the output:

array_47new.janet