V3 Migration Guide TODO

[gsteel]

This issue is for tracking content required in the migration guide for v2 -> v3

• Removal of AbstractFilter - Document why, and provide refactoring examples
• Final Everywhere, also tracked in v3 Docs: Improve documentation on change to final everywhere #119
• Addition of __invoke() to FilterInterface if not already covered
• Removal of AbstractSeparator in Remove AbstractSeparator and refactor descendants #194 needs documenting as per AbstractFilter
• Compress and Decompress refactor in Refactor/Replace Compression & Decompression Filters #195
• Filter Chain Changes in Refactor FilterChain #208