Start migrating rules away from counter system

[khiga8]

Relates to: #56

This PR starts prepping for deprecation of the counter system which we've used as a way to get around lack of support for inline disables. This is no longer necessary with the latest ERB version which now offers inline disable support.

This PR renames AvoidBothDisabledAndAriaDisabledCounter to be AvoidBothDisabledAndAriaDisabled, deprecating the counter from the name.

The to-be-deprecated counter system will continue to work if it's configured explicitly, so existing counter comments shouldn't break. Basically if ERBLint config is:

AvoidBothDisabledAndAriaDisabled:
  enabled: true
  enable_counter: true

The existing counter comments shouldn't break. We will default to counter system off. We can keep them enabled and turn them off as we migrate each rule.

This PR adds a doc which we will be surfaced during the new release which instructs how to migrate away from counters.
Preview migration doc 🔍 . I am hoping we can also include a script to ease this migration for our release.

If this approach seems fine, I plan to follow-up with every other rule to prep for migration internally.

[khiga8]

I will remove Counter from names in the README right before we cut release, for alignment with current version.

[khiga8]

@jonrohan what do you think of this counter deprecation approach? could it work for PVC too?

update: chatted in person and on board!

[accessibility-bot]

👋 Hello and thanks for pinging us! An accessibility first responder will review this soon.

• 💻 On PRs for our review: please provide a review environment with steps to validate, screenshots (with alt text), or videos demonstrating functionality we should be checking. This will help speed up our review and feedback cycle.
• ⚠️ If this is urgent, please visit us in #accessibility on Slack and tag the first responder(s) listed in the channel topic.

[owenniblock]

Excited for this change!