sdformat: Website schemas are old and URLs are not versioned?

## Environment
N/A

## Description
* Expected behavior: Latest schemas are visible via a clearly versioned mechanism.
* Actual behavior: Current schemas are old (e.g. http://sdformat.org/schemas/root.xsd), and are not versioned. Also, all remote assets that have cross-references require remote fetching. May inline.

## Steps to reproduce
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## Output
N/A