chore: Add make help command

[jkroepke]

Description

Minimalistic make help command to get the info with targets are availible without looking through the whole Makefile.

Project: stackit-sdk-go
Usage:
  make <target>

Targets:
  help                 show this help.
  lint                 Lint all code
  lint-golangci-lint   Lint Go code
  lint-scripts         Lint scripts
  project-help         Show help for the project
  project-tools        Install project tools
  sync-tidy            Sync and tidy dependencies
  test                 Run all tests
  test-go              Run Go tests
  test-scripts         Run tests for scripts

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs
  • Changelog in the root directory was adjusted (see here)
  • Changelog(s) of the service(s) were adjusted (see e.g. here)
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

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[jkroepke]

@marceljk fyi

[marceljk]

@jkroepke thanks for your contribution :) The PR looks good, only one nitpick in the comments. Afterwards the PR can be merged