Rewrite docs/development/schema.md and related components and patterns

[duncandewhurst]

This PR updates the old schema development page and related content to reflect the process followed and lessons learned from more recent standard development work.

Preview

• Closes Development (schema): Update extended JSON Schema section #39
• Closes Development (schema): Add an example to serializations section #38
• Closes Components (conversion tools): Add content about Flatten Tool #36
• Closes Development (schema): Update deprecation section #23

The new and updated content links to the following new repositories and tools, which should be reviewed at the same time:

• JSON Schema Extension
• Codelist Schema
• Standard Development Template (Airtable)

[kd-ods]

This all reads well. Just a few suggestions. I'm off to look at the linked tools now.

[kd-ods]

I've eye-balled the linked tools and added one issue/question to the JSON Schema extension repo. I can't say I spent ages, but it looks like they're good enough to be tried in practice, and we can improve them as we go!

Thanks so much for all this work, Duncan :-)

[jpmckinney]

The Airtable is very cool! I've cross-referenced the new repos from OCP's docs.

[duncandewhurst]

The Airtable is very cool! I've cross-referenced the new repos from OCP's docs.

Great. Thank you!