docs: explain the possibility of using system GHC, to reduce space requirements

A working setup was in exercism/haskell#1043:

1. Add to `~/.stack/config.yaml`:

```
# Stop downloading GHCs into isolated locations under ~/.stack.
install-ghc: false

# Allow Stack to pick the system GHC (false by default).
system-ghc: true

# Add the -dynamic flag to every invocation of GHC.
ghc-options:
"$everything": -dynamic
```

2. Check system GHC version and find a suitable resolver in https://www.stackage.org/
3. Modify stack.yaml to that resolver.

Add these instructions to https://github.com/exercism/haskell/blob/main/docs/INSTALLATION.md and https://github.com/exercism/haskell/blob/main/docs/TESTS.md as appropriate.

Will need to think about how to present them - is that something we're recommending to everyone, or is it just to those who want it? Will there be issues with students not being sure how to correctly follow the instructions, or whether they should even follow them at all? etc.