add docs about the matrix instance

[sareg0]

This is a draft for documentation about how to join our Matrix instance.

I used DeepL to do the translation, so need some careful review.

I don't know if we have a code of conduct, but if we do we could link it as a guide for how to conduct yourself in the instance.

Feedback welcome

[StrangeGirlMurph]

A button next to the signal and mastodon one in the header should also be added with the invite link: https://matrix.to/#/#community:matrix.heartofcode.org

A code of conduct is a great idea. We can copy one from similar communities. Please find one.

[sareg0]

A button next to the signal and mastodon one in the header should also be added with the invite link: https://matrix.to/#/#community:matrix.heartofcode.org

Addressed in 828e891

A code of conduct is a great idea. We can copy one from similar communities. Please find one.

Still working on finding a code of conduct

[StrangeGirlMurph]

Oh the guide is also missing the explanation of element. What element instance to use and how. Also a small part about matrix encryption would be nice maybe. I think that confuses people sometimes