Update documentation, examples, and release notes for client capability work

[dkropachev]

Summary

Update user-facing documentation, examples, and release notes for the client capability roadmap.

Problem

Several planned issues add or clarify advanced client behavior: helper lifecycle APIs, explicit routing fallback, transport configuration, TLS options, compression/header customization, and key-affinity semantics. Users need one coherent set of docs that explains how these pieces fit together without requiring them to read implementation details.

Required Documentation

Update README and examples for:

• when to use alternator.client(...)
• when to use create_client / create_resource
• when to use the future Helper / AsyncHelper
• host-only seeds and one shared port
• auth disabled by default and static credentials only
• explicit routing-scope fallback behavior
• topology validation helpers
• timeout and retry semantics
• connection pool settings that Python supports
• unsupported or limited transport knobs and closest alternatives
• TLS custom CA, mTLS, insecure trust-all warning, and key log file support
• request compression configuration
• header optimization configuration
• key route affinity modes and BatchWriteItem behavior
• node health status as planning-only/deferred unless a future implementation exists
• migration notes for deprecated names and changed defaults

Examples To Add Or Update

• helper lifecycle example
• explicit routing fallback example
• static auth example
• timeout/retry configuration example
• mTLS example if supported
• compression and header optimization example
• key affinity example with preloaded partition keys
• existing AWS SDK comparison example should stay current

Release Notes

Add release note guidance covering:

• additive compatible APIs
• deprecations
• behavior changes, if any
• migration steps
• versioning expectations

Acceptance Criteria

• README accurately reflects the implemented public API at the time this issue is closed.
• Examples run or are covered by syntax/lint checks where practical.
• Deferred node health remains described as planning-only unless a separate implementation has landed.
• Release notes mention compatibility impacts and migration paths.
• Documentation avoids relying on private attributes.
• make lint and example lint checks pass.

Related Code Areas

• README.md
• RELEASING.md
• examples/
• tests/unit/test_client_alias.py