docs: Update the contributing guidelines link.

[feanil]

We're moving towards a single set of guidelines org-wide.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (11e9e15) 76.29% compared to head (511ea74) 76.29%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #890 +/- ## ======================================= Coverage 76.29% 76.29% ======================================= Files 150 150 Lines 17117 17117 ======================================= Hits 13059 13059 Misses 4058 4058 ``` 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=openedx). 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=openedx)

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