Add first draft of a rule description style guide (#4386)

* Add first draft of a rule description style guide

* Fix a typo in the style guide draft

Co-authored-by: Brais Gabín <braisgabin@gmail.com>

* Update style guide to cover the three types of descriptions

* Discourage file names in code smell messages

Co-authored-by: Brais Gabín <braisgabin@gmail.com>

* Fix two minor mistakes in the style guide

* Explain the example on usage-aware recommendations

* Reference the guide on authoing rule descriptions

* Write detekt in lowercase

* Add an empty line after every heading

Co-authored-by: M Schalk <30376729+schalkms@users.noreply.github.com>

* Clean up the contribution guide

* Incorporate latest review comments into the style guide

* Fix a typo in the style guide

* Use consistent line breaks in the style guide

* Merge the style guide into the contribution guide

* Delete the standalone style guide

* Remove redundant space

Co-authored-by: Brais Gabín <braisgabin@gmail.com>
Co-authored-by: M Schalk <30376729+schalkms@users.noreply.github.com>