Adds Contributor Documentation

[swainn]

Description

This pull request adds documentation for contributors to Tethys Platform.

Draft:

• Introduction / Landing Page (contribute.rst)
• Contributing Code Overview (contribute/code.rst)
  • Getting Started Tutorial (contribute/code/getting_started.rst)
  • Setting up a development environment (contribute/code/dev_environment.rst)
  • Development Process (contribute/code/development_process.rst)
  • Testing (contribute/code/testing.rst)
  • Managing Issues (contribute/code/issues.rst)
  • Continuous Integration (contribute/code/continuous_integration.rst)
  • Maintaining Dependencies (contribute/code/dependencies.rst)
  • Coding Principles (contribute/code/coding_principles.rst)
    • Tethys Core (contribute/code/coding_principles/core.rst)
    • Tethys CLI (contribute/code/coding_principles/cli.rst)
    • Tethys Compute (contribute/code/coding_principles/compute.rst)
    • Tethys Gizmos (contribute/code/coding_principles/gizmos.rst)
    • Tethys Layouts (contribute/code/coding_principles/layouts.rst)
    • Tethys Portal (contribute/code/coding_principles/tethys_portal.rst)
    • Code Organization (contribute/code/coding_principles/organization.rst)
  • Deploying New Versions (contribute/code/deploying_tethys.rst)
• Contributing Documentation (contribute/documentation.rst)
  • Building the Docs (contribute/documentation/building.rst)
  • Source Files (contribute/documentation/source.rst)
  • Read the Docs (contribute/documentation/readthedocs.rst)
  • Guidelines (contribute/documentation/guidelines.rst)
  • Demo (contribute/documentation/demo.rst)
  • I18n (contribute/documentation/i18n.rst)
• Supporting the Community (contribute/community.rst)

[coveralls]

coverage: 100.0%. remained the same
when pulling 86d47f3 on contributor-documentation
into 7913409 on main.