docs/releases/timeline: add numbers before phases

Adjust the timeline docs and release infrastructure template to have
numbers before the phases. This makes clearer where the next phase
begins in the long infrastructure issue template.

Signed-off-by: Oliver Smith <ollieparanoid@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8815>

docs/releases/timeline.md

@@ -5,7 +5,7 @@ release cycle starts. Get the current CW with `date +%V`. During the release
 cycle we try to follow this plan closely, but it can happen that we run
 over the planned CWs.
 
-## Pre-Build
+## 1. Pre-Build
 
 CW 21 <small>(2026-05-18 - 2026-05-24)</small>
 
@@ -20,7 +20,7 @@ Team:
 * Create the release branch in pmaports.
 * Start building of binary packages early, so we can get through it for sure.
 
-## Branch
+## 2. Branch
 
 CW 22 <small>(2026-05-25 - 2026-05-31)</small>,
 CW 23 <small>(2026-06-01 - 2026-06-07)</small>
@@ -34,7 +34,7 @@ Team:
 * Rebase the release branch on main once
 * Build binary packages and images (configure BPO for that)
 
-## Test
+## 3. Test
 
 CW 24 <small>(2026-06-08 - 2026-06-14)</small>
 
@@ -45,7 +45,7 @@ Maintainers:
 * Fix stuff that is broken by creating MRs against pmaports main and assigning
   the `backport-to-YY.MM` label
 
-## Release
+## 4. Release
 
 CW 25 <small>(2026-06-15 - 2026-06-21)</small>