Update community membership guidelines to include Contributor role

[willtsai]

This pull request updates the community-membership.md documentation to clarify contributor roles, responsibilities, and requirements in the Radius project. The changes improve the structure, readability, and inclusiveness of the document, making it easier for new and existing contributors to understand how to participate and advance within the community.

Role definitions and requirements:

• Added a clear definition for the Contributor role, outlining ways to contribute and specifying the requirement of at least one merged PR in a Radius repository.
• Expanded and clarified the descriptions and requirements for Member, Approver, and Maintainer roles, including expectations for activity, review responsibilities, and paths to advancement. [1] [2] [3] [4] [5]

Inclusivity and onboarding improvements:

• Improved onboarding guidance for new contributors, emphasizing that everyone is welcome and describing multiple ways to contribute beyond code, such as documentation and community support.

Document structure and readability:

• Reorganized the document by replacing HTML comments and section headers with clearer markdown headings and subheadings for each role, and by moving role-specific details into dedicated sections. [1] [2] [3] [4] [5] [6] [7]

Process clarifications:

• Clarified the processes for sponsorship, review, and advancement for members, approvers, and maintainers, including details about inactivity and removal procedures. [1] [2] [3]

Removed outdated or redundant content:

• Deleted obsolete HTML comments and acceptance/voting process details to streamline the document and focus on current practices. [1] [2]