Improve list of contributors

[vnmabus]

Improve list of contributors to use allcontributors schema. This provides several advantages:

• The list is now much prettier and it is shown as part of the documentation
• Updating the list is easier, thanks to a bot
• For each user, the particular types of contributions made are now shown
• We now recognize all contributors, not just the ones in the team

Please, if anyone wants a different avatar/link or considers that I misrepresented their contributions, just answer to this issue.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2101576) 85.86% compared to head (474fd89) 85.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #557 +/- ## ======================================== Coverage 85.86% 85.86% ======================================== Files 147 147 Lines 11655 11655 ======================================== Hits 10007 10007 Misses 1648 1648 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/GAA-UAM/scikit-fda/pull/557/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.