docs(deprecation policy): align lifecycle sections with fragment-first flow

The "Deprecation" and "Removal" sections still promised that we notify
users via the release upgrade guide, but the rewritten Process section
treats the deprecation.d fragment (rendered on the release page and the
deprecations index) as the canonical user-facing notification. Update
both lifecycle sections to match: the fragment / enacted entry is the
default; the upgrade guide remains an option for richer treatment when
warranted, not a per-deprecation requirement.

Author: pront

docs/DEPRECATION_POLICY.md

@@ -38,8 +38,9 @@ A configuration option or feature in Vector is marked as deprecated.
 
 When this happens, we will notify by:
 
-- Listing the deprecation in the Deprecations section of the upgrade guide for the release the deprecation was
-  introduced in. This will include instructions on how to transition if applicable.
+- Adding a [`deprecation.d/`](../deprecation.d/) fragment that lists the deprecation on the release page and on the
+  always-current [deprecations index](https://vector.dev/deprecations/), including migration guidance. The release's
+  upgrade guide may also call out the deprecation when it warrants a richer treatment than the fragment provides.
 - Adding a deprecation note to the [documentation site][configuration] alongside the configuration or feature being
   deprecated.
 - When possible, output a log at the `WARN` level if Vector detects deprecated configuration or features being used
@@ -57,8 +58,10 @@ A deprecated configuration option or feature in Vector is removed.
 
 When this happens, we will notify by:
 
-- Listing the removal in the Breaking Changes section of upgrade guide for that release. This will include directions on
-  how to transition if applicable.
+- Recording the removal as an enacted entry in the deprecations data (via `cargo vdev deprecation enact`), which moves
+  the entry from the active list to the past-deprecations section on the [deprecations index](https://vector.dev/deprecations/)
+  and surfaces it on the release page. The release's upgrade guide may also call out the removal under Breaking Changes
+  when it warrants a richer treatment.
 
 When possible, Vector will error at start-up when a removed configuration option or feature is used.