Update the spec page on the research website

[drskalman]

Spec related changes

• [ ] General intro to Polkadot Spec: What is it all about.
• [ ] General intro to Spec team and its work.
• [ ] Intro into testing harness developed by the spec team.
• [ ] An intro into host spec (publication on the page).
• [ ] Put spec team members under publications in alphabetic order

General changes

• [ ] Remove redirect to empty overview page
• [ ] Link to forum in menu and under publications
• [ ] Remove or extend multisig page
• [ ] Move all "todos" to issue tracker

[FlorianFranzen]

We should also add the spec team to the members page.

[FlorianFranzen]

The structure for this was added in w3f/research#114, I will add the content in the weeks to come.

[lamafab]

@FlorianFranzen is working on this with @jonasW3F -> fix broken links, improve CI, etc. @FlorianFranzen will prioritize the overview paper first, though.