Docs: add instructions for Slang language server

[jwortmann]

1. Add instructions for Slang language server. I noticed that there is also a helper package for this server at https://github.com/LDAP/LSP-slangd, but that package is not on Package Control, uses the old AbstractPlugin, installs an older version of the language server, and it looks like it doesn't provide any additional functionalities. So for now we can add the manual config.
2. Removed zero-width space in YamlPipelines name (I assume that was accidental).
3. Replaced old https://packagecontrol.io/ links with the new Package Control website https://packages.sublimetext.io/

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[rwols]

LGTM. I would prefer these commits to be rebased instead of being squash-merged. I think changing the links deserves a separate atomic change, as well as the removal of the non-breaking space characters. But up to you.

[rwols]

By the way… I once again encourage you to join the Discord server. It helps with alignment on where things go.

[jwortmann]

I would prefer these commits to be rebased instead of being squash-merged.

Sure, cleaned up the commits a bit for that now.

By the way… I once again encourage you to join the Discord server. It helps with alignment on where things go.

Okay, I'll consider it... ;) but no guarantee that I will use it regularly.