chore: even better feature flag documentation (#22811)

Include the following changes:

- Include and require a section on restoring old behavior (or dealing with the required source changes) on removed/removable flags.
- Reorganize the Markdown file to put all tables up top, with links to the detailed flag information at the bottom.
- Include v1-only flags (including removed ones) in the details.
- Remove the "value" column from a couple of tables to make it fit GitHub's page width better


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

CONTRIBUTING.md

@@ -977,6 +977,7 @@ Adding a new flag looks as follows:
    your description, be sure to cover the following:
    - Consciously pick the type of feature flag. Can the flag be removed in a future major version, or not?
    - Motivate why the feature flag exists. What is the change to existing infrastructure and why is it not safe?
+   - In case of a "default change flag", describe what the user needs to do to restore the old behavior.
 4. Add an entry for your feature flag in the [README](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/cx-api/README.md) file.
 5. In your tests, ensure that you test your feature with and without the feature flag enabled. You can do this by passing the feature flag to the `context` property when instantiating an `App`.
    ```ts