doc: put landing specifics in details tag

Showing all the steps isn't usually useful and makes it seem like the
collaborator is meant to follow the manual steps. I've seen this happen
during at least one onboarding. The section is also a bit long to
comfortably scroll over quickly to find the next section.

Let's put those steps in a details block to make it more obvious that it
is there for unusual situations only.

Co-authored-by: Shelley Vohr <codebytere@gmail.com>

PR-URL: #35296
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

Author: Trott

doc/guides/collaborator-guide.md

@@ -469,6 +469,12 @@ code. If you wish to create the token yourself in advance, see
 
 ### Technical HOWTO
 
+Infrequently, it is necessary to manually perform the steps required to land a
+pull request rather than rely on `git-node`.
+
+<details>
+<Summary>Manual Landing Steps</Summary>
+
 Clear any `am`/`rebase` that might already be underway:
 
 ```text
@@ -626,6 +632,8 @@ your pull request shows the purple merged status,
 add the "Landed in \<commit hash>..\<commit hash>" comment if you added
 more than one commit.
 
+</details>
+
 ### Troubleshooting
 
 Sometimes, when running `git push upstream master`, you might get an error