docs: document CR scope, notifications and non-notification events

[SahilJat]

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Closes #7505
The Change Requests docs page had two gaps that were causing avoidable
support conversations:

1. Scope was not documented — the page said "attempting to change a flag
   value will prompt you to create a new Change Request" without clarifying
   that identity-level overrides are intentionally excluded from the workflow
   and go live immediately.

2. Notifications were not documented — the page mentioned assignee emails
   only once in passing. The full set of emails that fire (pending-approval to
   assignee, approval-granted to author, group assignment notification) was
   not described anywhere. Equally important, what does not trigger a
   notification (commit, deletion, webhooks) was also undocumented, leading
   customers to expect behaviour that does not exist.

**Changes made **

• Added a Scope of Change Requests section with a table showing which
  feature state types require a CR and which do not, with an explanation of
  why identity overrides are excluded (they do not participate in the
  versioning system the CR workflow is built on)
• Added a tip recommending segment-based targeting for teams that need
  approval gates on all changes
• Added a Notifications section with a table of all email notifications
  that fire during the CR lifecycle, and an explicit list of events that do
  not trigger notifications (commit, deletion, webhooks)
• Updated the "Create a Change Request" section to clarify that the prompt
  applies to environment defaults and segment overrides, not all flag changes

Both additions are verified against the actual implementation in
api/features/workflows/core/models.py.

How did you test this code?

This is a documentation-only change — no code was modified.

Verified the described behaviour against the source code:

• Scope — confirmed via FeatureState.type property and is_live
  property in api/features/models.py that identity overrides bypass
  versioning and are live immediately
• Notifications — confirmed via send_email_notification_to_assignee,
  send_email_notification_to_author, and
  notify_group_of_change_request_assignment hooks in
  api/features/workflows/core/models.py that the documented emails are the
  complete set
• Non-notifications — confirmed no email fires on commit, no rejection
  flow exists, and no CR lifecycle webhook events are present in the codebase

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