doc/manual: Add 'Debugging Nix' section

[michaelvanstraten]

This PR adds a new 'Debugging Nix' section to the Nix manual. It provides instructions on how to build Nix with debug symbols and how to debug the Nix binary using debuggers like lldb.

Motivation

Currently, it's hard to get started with launching a debuggable instance of Nix. Building Nix with debug symbols requires knowledge of internal environment variables like mesonBuildType, which are not well-documented.

Context

Please see #11502.

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