Restore storybook-addon-links

[shilman]

What happened

1. We consolidated all packages into the monorepo
2. We marked all of the original package repos as deprecated
3. https://github.com/storybooks/storybook-addon-links did not get transitioned to the monorepo
4. I read a stale PR and assessed that addon-links was deprecated
5. I started to remove addon-links from the docs and from the code, e.g. Remove all references to addons-links #1015

However, now as far as I can tell, the only problem was that addon-links was not properly ported to the monorepo.

What needs to happen

• addon-links ported to monorepo and tested
• documentation updated to refer to addon-links as a standalone package
• "built-in" actions and linkTo API should be deprecated (with 3.0?)

[thani-sh]

Version 3.0 is a good time to remove actions and linkTo APIs from core. But let's add a warning log in when using these APIs and make sure it continues to work for now. It'll be no longer supported from v3.0 and we can remove it completely at a later version. What do you think?

[ndelangen]

I should merge https://github.com/storybooks/storybook-addon-links into the monorepo?