docs: remove duplicate content from README and point to website

[Champbreed]

Removes duplicated content from the community README that now exists
on the OSCAL Compass website (https://oscal-compass.dev/community/).
The README is now specific to this repository and points to the
website for full project information, community meetings, and
project descriptions.

Closes #118

cc: @vikas-agarwal76 @degenaro

[Champbreed]

Note: Community meetings, email, and Slack details were removed as
they exist on the website. Would add them back if preferred.

[Champbreed]

While working on this issue, I noticed that docs/community.md in
the oscal-compass.github.io repo is manually maintained separately
from this README, which means both files still need to be updated
independently.

A better long-term solution would be to use the
mkdocs-include-markdown-plugin to have the website automatically
pull content directly from this README, eliminating the duplication
entirely rather than just reducing it.

Would the maintainers be open to a follow-up PR to implement this?

[degenaro]

@Champbreed Yes. Avoiding duplication is to strive for. Thx!