Move children schema to strands remove use of deprecated module

Contents (#112)

Operations

• Add ReadTheDocs config file
• Fix codecov upload
• Use reusable workflows
• Test with python3.9 too
• Upgrade actions

Dependencies

• Fix documentation requirements

Refactoring

• Move children and manifest schemas to Strands

Fixes

• Fix accordion usage in docs
• Swap deprecated/removed pkg_resources for importlib