Add documentation for extensions

[dalepotter]

Include links to known extensions (France / USA), and best practices, experiences with generating extensions.

[dalepotter]

This issue might better sit in the IATI-Extra-Documentation (under the current documentation structure) with the source for the current page on namespaces and extensions being here: https://github.com/IATI/IATI-Extra-Documentation/blob/version-2.03/en/namespaces-extensions.rst
Also linking to IATI/IATI-Guidance#21 which also touched on this issue.

Regardless of where this issue sits, it might be worth adding to this page. Whilst IATI/IATI-Extra-Documentation@243283c added extra context on the use of 'namespaced' element fields, there is currently no explicit reference that IATI Standard attributes can also be namespaced.

It might also be worth highlighting known/commonly-used extensions (as identified by @rolfkleef in a Discuss post) as real-world usage examples. Citing these publishers may also help from a data use perspective, since it identities publishes who may be providing information supplementary to the IATI Standard fields.

[samuele-mattiuzzo]

Thanks Dale!

[andylolz]

Extensions are currently listed on this dashboard page, and they add quite a lot of noise.

One way to highlight extension usage might be to separate them out on the dashboard, and then point to an extension-specific page.

This avoids pointing to individual publishers manually, which I guess would involve maintaining a curated list.