Simplify creation of a custom ruleset

[paul-dingemans]

Description

Simply creation of a custom ruleset

• Simplify and document the build file so that it is more convenient to be used to start a new project for a custom ruleset
• Modify the ktlint version in the build file during the release process
• Improve documentation
• Remove outdated comment in NoVarRuleTest.kt

Closes #3048

Checklist

Before submitting the PR, please check following (checks which are not relevant may be ignored):

• Commit message are well written. In addition to a short title, the commit message also explain why a change is made.
• At least one commit message contains a reference Closes #<xxx> or Fixes #<xxx> (replace<xxx> with issue number)
• Tests are added
• KtLint format has been applied on source code itself and violations are fixed
• PR title is short and clear (it is used as description in the release changelog)
• PR description added (background information)

Documentation is updated. See difference between snapshot and release documentation

• Snapshot documentation in case documentation is to be released together with a code change
• Release documentation in case documentation is related to a released version of ktlint and has to be published as soon as the change is merged to master