[doc] add link to rx example in Adapt Intents doc

[pasniak]

Add link to https://github.com/krisgesling/regex-example-skill/ (or its fork)
in Adapt Intents doc https://mycroft-ai.gitbook.io/docs/skill-development/user-interaction/intents/adapt-intents#vocab-.voc-files

[krisgesling]

Hi there, glad you found it useful and thanks for the suggestion to add it to the docs.

I've just nudged some Adapt guru's to take a quick look and make sure it reflects current best practice before we promote it as the way to do things.