spec: ldml-docs ✍

"comprehensive documentation on the new LDML keyboard format for keyboard authors"

- There is already the LDML spec - but we need to make a new reference, focussed on authors.
- And i think we should have a **Guide** to the language.
- how to get started, etc.
- Also there should be a guide (what to call it?) for the process of creating a file, using the tools, deploying, etc, under the Developer help in this keyman/developer/docs/help
- We should focus on desktop for now since that is what is usable, but describe touch as well.

- Pages will be in .md, under help.keyman.com/developer/ldml:
- /developer/ldml/reference
- /developer/ldml/guide

Compare to KMN:
- [KMN Reference](https://help.keyman.com/developer/language/reference/)
- [KMN Guide](https://help.keyman.com/developer/language/guide)

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- [ ] stub pages in the guide
- [ ] stub pages in the reference
- [ ] need links to glossary in the guide, as the user goes through, but with inline referenes