Add searchable snapshot API examples

[lcawl]

Relates to #2482

This PR copies examples and descriptions from https://www.elastic.co/guide/en/elasticsearch/reference/master/searchable-snapshots-apis.html.
It also adds doc_id values in preparation for redirects.

[github-actions]

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

API	Status	Request	Response
searchable_snapshots.cache_stats	⚪	Missing test	Missing test
searchable_snapshots.clear_cache	🟢	1/1	1/1
searchable_snapshots.mount	🟢	4/4	4/4
searchable_snapshots.stats	🟢	1/1	1/1

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

[github-actions]

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

API	Status	Request	Response
searchable_snapshots.cache_stats	⚪	Missing test	Missing test
searchable_snapshots.clear_cache	🟢	1/1	1/1
searchable_snapshots.mount	🟢	4/4	4/4
searchable_snapshots.stats	🟢	1/1	1/1

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

[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-3459-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 fd32ef05ff46091c0a91be83866f2430b2e6229d
# Push it to GitHub
git push --set-upstream origin backport-3459-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-3459-to-8.x.

[github-actions]

The backport to 8.17 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-8.17 8.17
# Navigate to the new working tree
cd .worktrees/backport-8.17
# Create a new branch
git switch --create backport-3459-to-8.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 fd32ef05ff46091c0a91be83866f2430b2e6229d
# Push it to GitHub
git push --set-upstream origin backport-3459-to-8.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.17

Then, create a pull request where the base branch is 8.17 and the compare/head branch is backport-3459-to-8.17.

[lcawl]

💚 All backports created successfully

Status	Branch	Result
✅	8.x

Questions ?

Please refer to the Backport tool documentation

[lcawl]

💚 All backports created successfully

Status	Branch	Result
✅	8.17

Questions ?

Please refer to the Backport tool documentation