Add help target to the makefile

[philderbeast]

Fix #10596. Add help for some of the makefile targets.

$ make help
all                            Build the Cabal libraries and cabal-install executables.
checks                         Run all local checks; whitespace, style, and lint.
doctest-install                Install doctest tool needed for running doctests.
doctest                        Run doctests.
exe                            Builds the cabal-install executables.
fix-whitespace                 Run fix-whitespace in fix mode.
ghcid-cli                      Run ghcid for the cabal-install executable.
ghcid-lib                      Run ghcid for the Cabal library.
help-banner                    Show the help banner.
help                           Show the commented targets.
init                           Set up git hooks and ignored revisions.
lib                            Builds the Cabal libraries.
lint-json                      Run HLint in JSON mode.
lint                           Run HLint.
style-commit                   Run the code styler on the previous commit.
style-modified                 Run the code styler on modified files.
style                          Run the code styler.
tags                           Generate editor tags, vim ctags and emacs etags.
users-guide                    Build the users guide.
users-guide-requirements       Install the requirements for building the users guide.
whitespace                     Run fix-whitespace in check mode.

---

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.