Examples: stabalise README format and create-next-app usage

[jthegedus]

In the same vein as #4002

• add create-next-app in example READMEs that were missing
• add READMEs for examples without them
• add yarn alternative instructions in each example's README - some examples were using yarn, others npm, this update just adds equal usage throughout the examples to be consistent.
• add note to firebase-hosting example indicating to tag me in Firebase related issues.

note: if the yarn stuff is an issue, I can remove this pretty simply, although it's only a few extra lines and just makes it clear that most examples can be used with either package manager 🤷‍♀️

this sed command was used to mass-replace:

for d in *
do
    ( cd $d && sed -n -i '1h;1!H;${g;s/bash\nnpm install\nnpm run dev\n/bash\nnpm install\nnpm run dev\n# or\nyarn\nyarn dev\n/;p;}' README.md)
done

[timneutkens]

This is amazing 😌