Ignore haddocks

[philderbeast]

I tried the cabal haddock-project command. It dumped thousands of untracked files into the source tree for haskell/cabal. This PR ignores that folder.

$ tree haddocks
haddocks
├── array
│   ├── array.haddock
│   ├── array.txt
│   ├── Data-Array-Base.html
...
    ├── synopsis.png
    ├── transformers.haddock
    └── transformers.txt

67 directories, 3487 files

There's the --output[=DIRECTORY] option but haddocks is the default directory for the output.

$ cabal haddock-project --help
Generate Haddocks HTML documentation for the cabal project.

Usage: cabal haddock-project [FLAGS]
   or: cabal haddock-project COMPONENTS [FLAGS]

Flags for haddock-project:
...
 --output[=DIRECTORY]           Output directory
...

---

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[philderbeast]

Label merge+no rebase is necessary when the pull request is from an organisation.