Add details on doing a modification for the documentation by yourself

[fharper]

Add details on doing a modification for the documentation by yourself.

It was shared a couple of times on Slack, and it's super straightforward, but still, we should put the step in the devrel repo.

It's just about doing a PR to documentation, but maybe we can details the process after, like you review, I review, we merge and it automatically sync.

[fharper]

done at https://github.com/mindee/devrel/blob/main/content/writing/documentation.md