Replace Slack links for Mattermost links

[santisoler]

Update links in README.rst and docs.

[codecov[bot]]

Codecov Report

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

Project coverage is 85.12%. Comparing base (ee53b66) to head (62f14ee).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #351 +/- ## ======================================= Coverage 85.12% 85.12% ======================================= Files 36 36 Lines 8408 8408 ======================================= Hits 7157 7157 Misses 1251 1251 ```

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