Fix flexbox settings for HDA flowsheet

[dangunter]

Fixes #142

For reasons unknown, this flowsheet needs a little extra CSS help, so custom.css adds a couple of flexbox settings that really should be the default (and seem to be elsewhere) to the main container for the notebook on the page.

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📚 Documentation preview 📚: https://idaes-examples--152.org.readthedocs.build/en/152/

[ksbeattie]

Any chance this comment could made a little clearer? Think of the poor soul who looks at this months or years from now...

[ksbeattie]

LGETM