Document woff conversion using `pyftsubset` in slides

[Manvel]

pyftsubset is also support woff2 creation and can be added into the subset extraction step, it does require brotli.

pyftsubset source-hans.ttf --unicodes=U+4E00-9FFF --flavor=woff2 --output-file=newfile.woff2

Reference: https://github.com/fonttools/fonttools#optional-requirements