[docs] Fix external links

[colleenmcginnis]

Fix external links that incorrectly use .md instead of .html.

[github-actions]

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• Rebuild this PR and all Elastic docs: run docs-build rebuild

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[marciw]

💚 All backports created successfully

Status	Branch	Result
✅	9.0

Questions ?

Please refer to the Backport tool documentation

[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-2826-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9ac0b0afdf80946135581d086ee9f4603f4af99e
# Push it to GitHub
git push --set-upstream origin backport-2826-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-2826-to-9.0.