docs: add README for cifar100 federated class-incremental learning example

[rajdhruvsingh]

Description

The cifar100 example has no README documentation at any level,
making it difficult for new users to understand what paradigm
it implements, what algorithms are available, and how to run it.

Proposed Change

Add examples/cifar100/README.md covering:

• Overview of federated class-incremental learning paradigm
• Dataset preparation
• Algorithm descriptions (FedAvg, GLFC, Fed-CI-Match, Fed-CI-Match-v2)
• Quick start guide
• Configuration parameters
• Directory structure

[rajdhruvsingh]

/assign