feat: add Documentation link to Admin Sidebar (fixes #3320)

[JorgeEscire]

References

Fixes #3320

Description

This PR adds a "Documentation" link to the Admin Sidebar menu. The link URL is automatically generated based on the DSpace backend version retrieved from the REST API root endpoint (e.g., DSpace 10.0.0 → https://wiki.lyrasis.org/display/DSDOC10x). No manual configuration is needed.

Additionally, this PR adds support for MenuItemType.EXTERNAL in AdminSidebarSectionComponent, which previously only supported internal routerLink navigation. External links now open in a new browser tab.

Instructions for Reviewers

List of changes in this PR:

• Created DocumentationMenuProvider that reads dspaceVersion from the REST API root endpoint, extracts the major version, and builds the documentation URL dynamically
• Modified AdminSidebarSectionComponent (.ts and .html) to support MenuItemType.EXTERNAL links with target="_blank" rendering
• Registered the new provider in app.menus.ts under MenuID.ADMIN
• Added i18n keys menu.section.documentation in en.json5 and de.json5
• Includes error handling: if the API fails or the version cannot be parsed, falls back to https://wiki.lyrasis.org/display/DSDOC/

How to test:

• Log in as a Site Administrator
• Verify the "Documentation" link appears at the bottom of the Admin Sidebar with a book icon
• Click on "Documentation" and verify it opens the correct DSpace documentation wiki in a new tab (e.g., https://wiki.lyrasis.org/display/DSDOC10x for DSpace 10.x)
• Verify the link URL matches the major version of your DSpace backend

Checklist

• My PR is created against the main branch of code
• My PR is small in size
• My PR passes ESLint validation using npm run lint
• My PR doesn't introduce circular dependencies
• My PR includes TypeDoc comments for all new public methods and classes
• My PR passes all specs/tests
• My PR aligns with Accessibility guidelines
• My PR uses i18n keys instead of hardcoded English text
• My PR includes details on how to test it
• If my PR includes new features or configurations, I've provided basic technical documentation in the PR itself