Adds security and show&tell notes to documentation

[kellyguo11]

Description

Adds a security page for indicating process on reporting security issues.
Also adds a note in readme to point users to the Show & Tell section of the discussion board to share projects and tutorials.

Type of change

• This change requires a documentation update

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there