DOCS: Convert Painless diagrams to mermaid

[kilfoyle]

This updates four diagrams in our Painless docs to use Mermaid. The originals, in png format, were really hard to read given the large amount of text. The Mermaid versions render much more clearly and with a "View fullscreen" icon for better usability.

Closes: elastic/docs-content#4683

[github-actions]

🔍 Preview links for changed docs

• docs/reference/scripting-languages/painless/how-painless-dispatches-function.md
• docs/reference/scripting-languages/painless/painless-language-specification.md
• docs/reference/scripting-languages/painless/painless-operators.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

When to use applies_to tags:

✅ At the page level to indicate which products/deployments the content applies to (mandatory)
✅ When features change state (e.g. preview, ga) in a specific version
✅ When availability differs across deployments and environments

What NOT to do:

❌ Don't remove or replace information that applies to an older version
❌ Don't add new information that applies to a specific version without an applies_to tag
❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
• Reach out in the #docs Slack channel

[elasticsearchmachine]

Pinging @elastic/core-docs (Team:Docs)

[yetanothertw]

LGTM, thank you! 📊