x-pack/filebeat/input/cel: mention mito tool

[efd6]

What does this PR do?

Adds a link to the mito CEL playground tool.

Why is it important?

The tool makes developing CEL programs easier by decoupling from the stack. So this will improve UX.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Use cases

Screenshots

Logs

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2023-03-29T01:04:19.632+0000

• Duration: 8 min 24 sec

❕ Flaky test report

No test was executed to be analysed.

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b cel-mito-doc upstream/cel-mito-doc
git merge upstream/main
git push upstream cel-mito-doc