Feat: Documents Uppy file metadata configuration

[corovcam]

Related PRs: inveniosoftware/invenio-rdm-records#2288, inveniosoftware/invenio-app-rdm#3386

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Description

Adds documentation for configuring file metadata fields within the Uppy uploader interface. This new section details how users can add custom metadata (e.g., captions, descriptions, custom fields) directly during the file upload process, with support for validation and required fields.

The documentation outlines:

• The properties for defining metadata fields (e.g., id, name, defaultValue, render, condition).
• An example demonstrating standard text inputs, custom rendered checkboxes, and hidden dynamic fields.
• Two methods for applying the configuration: passing as a prop to the UppyUploader component or extending default configurations.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've targeted the master branch.
• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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[fenekku]

Postponed for after v14.0 release (not adding it to v14 board to reduce noise right now).

[ntarocco]

@fenekku can we add this to the v14 doc?

[fenekku]

@ntarocco

@fenekku can we add this to the v14 doc?

I'd say to wait on it given the documentation is related to 2 open PRs with unresolved comments and points to "default configuration" in the oarepo fork of invenio-rdm-records. Since we have passed feature freeze for v14 and there is a lot of other features/contributions more clearly done to review/test/document than this one, we can put this one aside. Happy to get it in as 14.1 though!