wip on migration fo extension basics

docs/extend.md

docs/extensions/basics.md

docs/extensions/civix.md

docs/extensions/config.md

docs/extensions/create-page.md

docs/extensions/custom-api.md

docs/extensions/custom-reports.md

docs/extensions/custom-searches.md

docs/extensions/customize-screens.md

docs/extensions/files.md

docs/extensions/payment-processor.md

docs/extensions/permissions.md

docs/extensions/storing-data.md

docs/extensions/using-hooks.md

mkdocs.yml