GitHub: remove "Additional documentation" section from PR template

Reasons for removing it:

- Having to deal with this part of the PR template is additional work and/or
  causes confusion ("I initially put the release note in the docs section by
  mistake.").
- Contributors typically don't know what to put there, reducing its usefulness.
- When contributors stop reading past that section they miss the AI disclosure section.

My subject impression is that this section is not used or not used correctly.
To verify, I ran an analysis of the PRs that went into 1.37.

- No docs code block at all: 497 PRs (43.4%) — e.g. http://github.com/kubernetes/kubernetes/pull/136271
- Empty docs block (blank/only comment): 453 PRs (39.5%) — e.g. http://github.com/kubernetes/kubernetes/pull/117119
- docs block = only a placeholder (N/A/None/-): 105 PRs (9.2%) — e.g. http://github.com/kubernetes/kubernetes/pull/129079
- docs block = a single KEP link: 64 PRs (5.6%) — e.g. http://github.com/kubernetes/kubernetes/pull/134639
- docs block = multiple KEP links, nothing else: 15 PRs (1.3%) - e.g. http://github.com/kubernetes/kubernetes/pull/138759 (three KEP links)
- docs  block = KEP link(s) plus other non-KEP info (Usage docs, etc.): 5 PRs (0.4%) — e.g. http://github.com/kubernetes/kubernetes/pull/140052 (KEP + sysctl Usage doc)
- docs  block = other info, no KEP link at all: 7 PRs (0.6%) - e.g. http://github.com/kubernetes/kubernetes/pull/140033 (note about a docs typo fix)

This was first suggested on
Slack (https://kubernetes.slack.com/archives/C2C40FMNF/p1788427548615859) two
weeks ago, with no objections and several people supporting the proposal.

.github/PULL_REQUEST_TEMPLATE.md

@@ -60,27 +60,6 @@ For more information on release notes see: https://git.k8s.io/community/contribu
 
 ```
 
-#### Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:
-
-<!--
-This section can be blank if this pull request does not require a release note.
-
-When adding links which point to resources within git repositories, like
-KEPs or supporting documentation, please reference a specific commit and avoid
-linking directly to the master branch. This ensures that links reference a
-specific point in time, rather than a document that may change over time.
-
-See here for guidance on getting permanent links to files: https://help.github.com/en/articles/getting-permanent-links-to-files
-
-Please use the following format for linking documentation:
-- [KEP]: <link>
-- [Usage]: <link>
-- [Other doc]: <link>
--->
-```docs
-
-```
-
 #### AI usage disclosure:
 
 <!--