[OpenAPI] Remove self-referential externalDocs

[lcawl]

This PR accomplishes the first step of #2748 (i.e. no longer adding externalDocs in the OpenAPI operations that are self-referential.

It does not accomplish the second step, which is to enact some way to add optional externalDocs that are meaningful from an API docs point of view. For example, a link to https://www.elastic.co/guide/en/elasticsearch/reference/current/analysis.html from the _analyze API.

[l-trotta]

LGTM!

[github-actions]

The backport to 8.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.x 8.x
# Navigate to the new working tree
cd .worktrees/backport-8.x
# Create a new branch
git switch --create backport-2925-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 e7ffee1ba8b2972cc4a43dd0a83b731225684150
# Push it to GitHub
git push --set-upstream origin backport-2925-to-8.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.x

Then, create a pull request where the base branch is 8.x and the compare/head branch is backport-2925-to-8.x.