docs(ocr): Add docs entry for OnnxTR OCR plugin

[felixdittrich92]

Adds documentation entry for the OnnxTR OCR plugin

ref.: docling-OCR-OnnxTR

Overrides #1222

Issue resolved by this Pull Request:
Resolves #1209

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

[mergify]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🟢 Enforce conventional commit

Wonderful, this rule succeeded.

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[felixdittrich92]

@cau-git @dolfim-ibm

[cau-git]

@felixdittrich92 Many thanks! great to see this working as a plugin.