Update eLabFTW file source template docs

[kysrpex]

Include the template itself and a screenshot of the form generated by the template in the docs, just like it is done for the other templates.

Follow up of #19493, these docs were missing in the original PR.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Run make docs-develop.
  2. Navigate to file:///home/you/Documents/Repositories/galaxy/doc/build/html/admin/data.html#allow-users-to-define-elabftw-instances-as-file-sources

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[kysrpex]

@davelopez I realized this section was missing when reviewing #19619.

[davelopez]

Thanks!