docs: expand enrollment state machine into an enrollment lifecycle page

[jaredlockhart]

Because

• The enrollment page was sparse and did not answer the questions people actually ask about a live recipe: when is enrollment re-evaluated, what persists across syncs, does resizing a rollout capture previously not-selected clients, and does sticky enrollment affect bucketing.
• It described only the mobile SDK, even though the desktop client implements the same model.

This commit

• Adds an evaluation-sequence flowchart (feature conflict, app/channel, targeting, bucketing) and a "what persists across evaluations" table.
• Adds a "what changes when you edit a live recipe" table scoped to the edits Experimenter actually allows (resize a rollout, end enrollment, end the recipe), plus a note that targeting/branches/feature values are locked at launch and require a clone.
• Clarifies that sticky enrollment gates only targeting for already-enrolled clients, and does not affect bucketing or not-enrolled clients.
• Covers both clients and collects the mobile/desktop differences in one table.
• Renames the page to Enrollment Lifecycle and moves it under Platform Guides (file, id, and slug updated to match).

[yashikakhurana]

Yes very helpful, thank you @jaredlockhart