[DOCS] Add cloud trial link to reindex API description

[charlotte-hoblik]

Adds an inline link to the Elastic Cloud registration page on the reindex API's existing "Elastic Cloud Hosted" mention, in the context of reindexing from a remote cluster.

This addresses #752 by restoring a cloud trial entry point on a high-traffic API page that lost its cloud trial CTA after migrating to Bump.sh.

The other four pages listed in the issue (search, create, bulk, indices create) were reviewed but don't have a natural place for a cloud trial link -- they are basic CRUD operations with no existing cloud or licensing mentions. A platform-level solution (e.g., a header banner on Bump.sh pages) would be more appropriate for those.

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[leemthompo]

LGTM but please check with dev tools folks whether we update package-lock.json file or not, if not sure :)

[github-actions]

The backport to 9.2 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-9.2 9.2
# Navigate to the new working tree
cd .worktrees/backport-9.2
# Create a new branch
git switch --create backport-6181-to-9.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 42814cd94530b53e87a1630fa7ccd82488502510
# Push it to GitHub
git push --set-upstream origin backport-6181-to-9.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.2

Then, create a pull request where the base branch is 9.2 and the compare/head branch is backport-6181-to-9.2.