Wiki: Improve instructions for creating sticker packs

[aWeinzierl]

• update package names
• improve documentation for Windows users
• add setup heading
• add hint for homeserver URL

Updated wiki: https://github.com/aWeinzierl/stickerpicker-1/wiki/Creating-packs

[viasux]

Would be nice if EDITING packs was detailed too

[Oha-you]

Addition for Windows users: you also need to install python-magic-bin to get rid of the Magic error (as per python-magic docs)

[Oha-you]

Also note:

• You don't actually need to specify "sender": "@you:matrix.server.name" when sending an m.widgets event. At least from my own exprience with latest Synapse/Element versions; the sender of the event is added automatically and is visible in Source
• You should click "Send Account Data", not "Send Custom Event"
• It doesn't really matter whether or not m.widgets already exists. So it's easier to ask users to just "Send Account Data"