Update CONTRIBUTING.rst to point to website

Background

Currently !796 (closed) just points to the source BuildStream Policies file, but once the website is finished we should point to that, to stop fragmentation.

Task description

• Point users to the BuildStream website instead of gitlab source files

Acceptance Criteria

---

mentioned in merge request !796 (closed)

Note, this information is about contributing, along with commit policies, labeling of issues, etc; so this will not point to the website, but rather the CONTRIBUTING.rst will get more structured and will consume the additional policies we need to add there.

There was some discussion about putting policies about committers and such separately on the website, but it was ultimately decided that this should all go in one place.

added To Do label

added Important label

changed milestone to %BuildStream_v1.4

closed