Populate fundamentals overview doc

[salmad3]

Done Criteria

• Provide a holistic overview of the primary primitives of libp2p.
• Page should resemble an index that includes introductory material and shapes how the reader would understand how the fundamentals of libp2p relate to each other.

Why Important

Page was created as a placeholder and requires additional material.

[salmad3]

Based on this PR which creates a "Core Abstractions" and "Protocols" section, the overview doc is no longer needed.