Fix wrong entry in fields.type_mapping.
Add fields.TYPE_MAPPING.

... and deprecate fields.type_mapping
docs: Add RTD Sphinx Theme to Acknowledgements.
docs: Fix typo.
docs: Change order of chapters.
docs: Add chapter "Advanced Topics"

With content previously in "Schema" chapter.
docs: Use correct capitalization of TYPE_MAPPING.
Update changelog.
docs: Document ordered output.
docs: exclude SchemaMeta.__prepare__
docs: Add field order recap.
Merge branch 'feature/docs' into develop