Move "getting started" info in README template to a central place

[timmc-edx]

Our repos often have a "how to develop on this repo" section in their README that describes the use of virtualenvs, git, and Open edX-specific workflows (such as make requirements). As a result, we have a large variety of READMEs with often outdated or incorrect information in this section.

A/C:

• Find or create a docs page for how to hack on an Open edX repo
  • If one already exists, use it
  • We may also want sub-pages for libraries and IDAs, etc.
  • Confirm with Axim that there isn't a better choice
  • ✅ In docs: Add how-to for Python development (virtualenvs, make test) docs.openedx.org#392 we've created https://docs.openedx.org/en/latest/developers/how-tos/get-ready-for-python-dev.html
• Update OEP-55 to request that READMEs link to this page rather than having a detailed Getting Started section
  • docs: Suggest linking to the developer how-to docs rather than inlining open-edx-proposals#536
  • docs: Tweak suggested heading a bit open-edx-proposals#539 (tweak to match cookiecutter)
• Update the cookiecutter README template(s) to link there instead of inlining the information
  • feat: Link to getting-started instructions rather than templating them #405
• Announce
• Optionally: Update the READMEs in a few high-use repos
  • docs: Replace getting-started instructions with link edx-django-utils#359
  • docs: Replace getting-started instructions with link openedx-events#289

Notes:

• Somewhere under https://docs.openedx.org/en/latest/developers/index.html is probably the best home for this