Update installation instructions

[jordiferrero]

Description of the change

Trying to install lumispy for someone else the other day, I encountered some issues that were not clear from the guide. Have added some clarifications.

Progress of the PR

• [ ] ready for review.

[codecov-commenter]

Codecov Report

Base: 98.36% // Head: 97.81% // Decreases project coverage by -0.54% :warning:

Coverage data is based on head (3cea691) compared to base (a3ac072). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #141 +/- ## ========================================== - Coverage 98.36% 97.81% -0.55% ========================================== Files 11 11 Lines 550 550 ========================================== - Hits 541 538 -3 - Misses 9 12 +3 ``` | [Impacted Files](https://codecov.io/gh/LumiSpy/lumispy/pull/141?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy) | Coverage Δ | | |---|---|---| | [lumispy/signals/cl\_spectrum.py](https://codecov.io/gh/LumiSpy/lumispy/pull/141/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy#diff-bHVtaXNweS9zaWduYWxzL2NsX3NwZWN0cnVtLnB5) | `94.91% <0.00%> (-5.09%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy)

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[jlaehne]

We still have the duplicate installation instructions in the user guide and in https://github.com/LumiSpy/lumispy/blob/main/INSTALLATION.md that should be merged together in the page of the user guide.