doc/manual: define {local,remote} store, binary cache, substituter

Nix veterans intuitively know what the following terms mean.  They are
used in several places in the nix documentation, but never defined:

- local store
- remote store
- binary cache
- substituter

In particular, I found the last two terms to be confusingly similar.
Let's give definitions for them.
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- [Garbage Collector Roots](package-management/garbage-collector-roots.md) - [Garbage Collector Roots](package-management/garbage-collector-roots.md)
- [Channels](package-management/channels.md) - [Channels](package-management/channels.md)
- [Sharing Packages Between Machines](package-management/sharing-packages.md) - [Sharing Packages Between Machines](package-management/sharing-packages.md)
- [Terminology](package-management/terminology.md)
- [Serving a Nix store via HTTP](package-management/binary-cache-substituter.md) - [Serving a Nix store via HTTP](package-management/binary-cache-substituter.md)
- [Copying Closures via SSH](package-management/copy-closure.md) - [Copying Closures via SSH](package-management/copy-closure.md)
- [Serving a Nix store via SSH](package-management/ssh-substituter.md) - [Serving a Nix store via SSH](package-management/ssh-substituter.md)

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# Terminology
A *local store* exists on the local filesystem of the machine where
Nix is invoked. The `/nix/store` directory is one example of a
local store. You can use other local stores by passing the
`--store` flag to `nix`.
A *remote store* is a store which exists anywhere other than the
local filesystem. One example is the `/nix/store` directory on
another machine, accessed via `ssh` or served by the `nix-serve`
Perl script.
A *binary cache* is a remote store which is not the local store of
any machine. Examples of binary caches include S3 buckets and the
[NixOS binary cache](https://cache.nixos.org). Binary caches use a
disk layout that is different from local stores; in particular, they
keep metadata and signatures in `.narinfo` files rather than in
`/nix/var/nix/db`.
A *substituter* is a store other than `/nix/store` from which nix will
copy the realisation of a derivation instead of building it. Nix will
not copy a realisation from a remote store unless one of the following
is true:
- the realisation is signed by one of the `trusted-public-key`s
- the substituter is in the `trusted-substituters` list
- the `no-require-sigs` option has been set to disable signature checking