Fix links to docs

[SebastianStehle]

It is not really a yjs bug but I think it should be fixed: The links to the documentation and forum are inconsistent and confusing:

Problems

1. In the repos the docs are linked:

2. In the github org the web page is linked:

3. The web page has confusing links:

• Documentation is pointing to github
• Discuss is pointing to the forum, but is not called forum as in the docs:
• Gitter is linking to the chat, but not called chat as in the docs.

Solution

1. Always link to the web page
2. Always call it forum
3. Always call it chat.
4. Never link to github for documentation