Update documentation to reflect TCP support

[ebkalderon]

Added

• Add doc tests for Server demonstrating initialization of both stdio and TCP transports.

Changed

• Split server example into separate stdio and tcp examples.
• Add mention of TCP support to README.md.
• Rename src/stdio.rs to src/transport.rs and update documentation to indicate its transport-agnostic nature.

Removed

• Remove unused initialization params from stdio and tcp examples.

CC @saskenuba, because I can't add you as a GitHub reviewer.

Closes #196.