Element for the first release

[lionelkusch]

This PR contains the documentation and the information for making a release.
It's not read and need to update from the first experience of making a release.

This was inspired from:
https://hackmd.io/JGsO9xm6TsKzKjm-thKnIQ
https://github.com/skrub-data/skrub/blob/main/RELEASE_PROCESS.rst
https://nilearn.github.io/stable/maintenance.html#how-to-make-a-release

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.29%. Comparing base (8100263) to head (ab76ff8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #258   +/-   ##
=======================================
  Coverage   99.29%   99.29%           
=======================================
  Files          24       24           
  Lines        1277     1277           
=======================================
  Hits         1268     1268           
  Misses          9        9           

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[bthirion]

atm we're still setting up things; We need to have a more homogeneous and many improvement before launching 1.0.0

[bthirion]

Suggested change

	Version 1.0.0
	Version 0.3.0
	```We're not ready for a 1.0.0 yet

[lionelkusch]

We can push a pre-release in this case.
For me, this is quite a major modification to the library because all the interfaces and methods have been modified.
However, for the moment, the modification is not yet finished.