Update documentation

[caufieldjh]

• Simplify the README and move most details to docs site
• Provide guide to building custom templates (including Is there a way to integrate a custom LinkML schema with the pip-installed version of the package? #222)
• Provide explanations for common errors + workarounds
• Describe functions (use examples from release notes)
• Include section for release notes, with script to auto-copy from each release. This may make a good GH action, or include it as part of the release action Future PR

[caufieldjh]

I'll add the copying of release notes to the docs in a near-future PR