Improve SEVIRI metadata documentation

[sfinkens]

Add an example showing how to access raw metadata.

Also, pin mamba version to latest release < 2 as a temporary workaround until the upstream issue with conda-incubator is fixed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.06%. Comparing base (9e3b342) to head (6a4f2af).
Report is 54 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2915   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files         373      373           
  Lines       54465    54406   -59     
=======================================
- Hits        52321    52267   -54     
+ Misses       2144     2139    -5     

Flag	Coverage Δ
behaviourtests	3.99% <ø> (+<0.01%)	⬆️
unittests	96.16% <ø> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 11108550483

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 96.164%

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Totals
Change from base Build 10957325874:	0.0%
Covered Lines:	52501
Relevant Lines:	54595

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💛 - Coveralls

[mraspaud]

LGTM