Add a runtime switch guide to control Impeller usage

[bc-lee]

Description of what this PR is changing or adding, and why:

Impeller has been causing issues on lower-end or older devices.
While stabilizing Impeller remains a priority, it is essential
to provide app developers with runtime control over its usage.
This allows decisions to be made on a per-device or per-SoC basis.

This patch introduces a guide for controlling Impeller usage at runtime
using the getFlutterShellArgs() method.

Issues fixed by this PR (if any):
Partially fixes flutter/flutter#160902

PRs or commits this PR depends on (if any):

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.