Adds an external link to Cluster allocation explain API spec

[szabosteve]

Overview

Related to https://github.com/elastic/docs-projects/issues/302 and elastic/docs-content#1698

DO NOT MERGE BEFORE elastic/docs-content#1698

This PR adds a link to the Cluster allocation explain API description that points to the more complex API example page in the Troubleshooting section.

[github-actions]

Following you can find the validation results for the API you have changed.

API	Status	Request	Response
cluster.allocation_explain	🟢	4/4	4/4

You can validate this API yourself by using the make validate target.

[github-actions]

Following you can find the validation results for the API you have changed.

API	Status	Request	Response
cluster.allocation_explain	🟢	4/4	4/4

You can validate this API yourself by using the make validate target.

[github-actions]

The backport to 9.0 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-9.0 9.0
# Navigate to the new working tree
cd .worktrees/backport-9.0
# Create a new branch
git switch --create backport-4523-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 d4d66dcfdff4d97b37ed729531260f9d274d2d80
# Push it to GitHub
git push --set-upstream origin backport-4523-to-9.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.0

Then, create a pull request where the base branch is 9.0 and the compare/head branch is backport-4523-to-9.0.