Update package README template from Agentless to Elastic Managed integrations

## Summary

The integration README scaffold still documents **Agentless deployment**. Existing packages were updated to **Elastic Managed enabled integration** wording and docs links in elastic/integrations#20987. New packages created with `elastic-package create package` still get the old Agentless section.

This was raised on that PR: https://github.com/elastic/integrations/pull/20987#discussion_r3903166728

## Source

`internal/packages/archetype/_static/package-docs-readme.md.tmpl` (currently around the Agentless commented block):

https://github.com/elastic/elastic-package/blob/2a1e840d0326fc949a0384eefc2afa1c03844736/internal/packages/archetype/_static/package-docs-readme.md.tmpl#L41-L47

## What to change

Replace the commented Agentless block with the Elastic Managed copy already used in integrations (for example `packages/filelog_otel/_dev/build/docs/README.md`).

cc: @mrodm

Open internal/packages/archetype/_static/package-docs-readme.md.tmpl and inspect the commented Agentless block around the referenced lines. Compare it with packages/filelog_otel/_dev/build/docs/README.md, then replace the scaffold wording and links so newly created packages use the Elastic Managed copy.

Written by the indexing model from the issue text.