(PUP-12064) Add task to generate function reference

[joshcooper]

bundle exec rake references:function

This task uses puppet strings to generate references/function.md.

The task requires the puppet-strings gems, so add it to the documentation
bundler group. Since the group is optional, check to see if the gem is present
before trying to load it, similar to how we handle ronn.

If there are both 3.x and 4.x functions with the same name, we ignore the 3.x
version and document the 4.x version.

This code is adapted from puppet-docs. The old code had a bug as it used to
replace /####\s/ with '###\s' resulting in a literal '\s' in the header for
some functions.

If puppet-strings gem is installed, then the man page generation will include
strings as a new application type. Since we don't actually ship strings
in puppet, hide it from man page generation.

[joshcooper]

Assuming you have the osp-docs repo checked out, you can verify the rake task generates the same result, ignoring the date and git ref:

$ bundle config set with documentation
$ bundle update
$ bundle exec rake references:function
...
$ diff --brief -I built_from_commit -I 'This page was generated from the Puppet source code' references/function.md ../osp-docs/function.md 
$