Use direnv and nix-direnv to automatically activate nix shell #1765

Background

In #1738 we addded support for nix shells in the MPC project with activation through nix develop.

Having to activate the shell manually every time is tedious, and we should make our setup activate the nix environment when opening the shell in the MPC repo directory.

direnv and nix-direnv are great tools for this purpose.

User Story

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Acceptance Criteria

Add instructions for using direnv and nix-direnv to the MPC project, such that the nix shell automatically activates in the repository.

Resources & Additional Notes

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