Spruce up organization metadocuments

This PR...

[x] Adds a CHARTER to the README.md of the repo (not the profile)
[x] Adds a better banner image to the profile/README.md
[x] Adds links to the profile/README.md
[x] Adds a SECURITY.md for the entire org (uses devcontainers.contrib+security@gmail.com as the email)
[x] Adds a CONTRIBUTING.md for the entire org
[x] Adds a SUPPORT.md for questions
[x] Adds a cute PR template document

devcontainers-contrib / .github