docs: mention foreman under hacking

Dependencies

• [2] BZQQZDPL doc: document how to run tests
• [3] 3JXCTKEC Add markdown files for documentation

Change contents

• replacement in doc/manual/src/hacking.md at line 1
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  − Hacking

  − =======
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  + # Hacking
• replacement in doc/manual/src/hacking.md at line 9
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  − To build it and its dependencies:
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  + To enter a shell in which all environment variables (such as PERL5LIB)

  + are set up so that those dependencies can be found:
• edit in doc/manual/src/hacking.md at line 12
  [3.7024]→[3.7024:7226](∅→∅)
  − $ nix-build release.nix -A build.x86_64-linux

  −

  − To build all dependencies and start a shell in which all environment

  − variables (such as PERL5LIB) are set up so that those dependencies can

  − be found:

  −
• replacement in doc/manual/src/hacking.md at line 20
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  − You can run the Hydra web server in your source tree as follows:
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  [3.7429]
  + You start a local database, the webserver, and other components with

  + foreman:

  +

  + $ foreman start

  +

  + You can run just the Hydra web server in your source tree as follows:
• edit in doc/manual/src/hacking.md at line 46
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  +

  + ## Building

  +

  + To build Hydra and its dependencies:

  +

  + $ nix-build release.nix -A build.x86_64-linux