Modify installation instructions to use `import appsignal`

[unflxw]

This is a sequel to #200 that I, uh, kinda forgot to do.

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Modify installation instructions

Add a warning emoji whenever manual instrumentation is required, that is, always.

For Django's instructions, refer to the modification needed as a generic "your entrypoints" and leave it for the docs to explain further.

Change other instructions to use import appsignal, replacing the use of from __appsignal__ import appsignal.

Specify requests dependency in hook

I am not sure exactly why this change is needed now. I think Hatch added support for specifying custom build hook dependencies directly in the custom build hook in the last release, and this might have accidentally broken the way in which we used to specify this dependency.

It might also be that the breakage is intentional, in which case they forgot to update the docs.

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