Document new language server architecture

[dbanck]

Description

The introduction of features and splitting of the internal state as part of #1667 changed some aspects of the language server architecture. We need to reflect these in the docs/

Links

• LS state & performance refactoring #1667

Help Wanted

• I'm interested in contributing a fix myself

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[dbanck]

Closed via #1740

[github-actions]

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