Remove doc references to setup.py

[mraspaud]

This PR removes outdated references to setup.py.

• [x] Closes #2782

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.94%. Comparing base (05fdcbf) to head (87d072d). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2786 +/- ## ======================================= Coverage 95.94% 95.94% ======================================= Files 379 379 Lines 53693 53693 ======================================= Hits 51515 51515 Misses 2178 2178 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2786/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [behaviourtests](https://app.codecov.io/gh/pytroll/satpy/pull/2786/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `4.09% <ø> (ø)` | | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2786/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.04% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

Pull Request Test Coverage Report for Build 8779757089

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 8779608590:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls