Improve build pipeline

[Mischback]

• some progress on Build pipeline #54
• close Update pre-commit configuration #125
• review Makefile:
  • add $(STAMP_NODE_READY) and $(TOX_VENV_INSTALLED) as order-only prerequisites as needed
  • FIXME: STYLE_DIR := $(THEME_DIR)/_src/style
  • FIXME: util/lint/prettier and util/lint/stylelint require $(STAMP_NODE_READY) as (order-only) prerequisite, as they are executed from the local NodeJS installation
  • remove development/debug-related echo statements
  • there are some places, where build artifacts are required for further processing (e.g. .build/_static/style.css). All files in .build are created/updated when running Sphinx / when the recipe for $(STAMP_SPHINX_COMPLETED) is executed. Is it safe to include an empty target like $(BUILD_DIR)/_static/style.css : $(STAMP_SPHINX_COMPLETED) ; in the Makefile to track this dependency? make's manual explicitly mentions the use case for artifacts, that are generated as side-effects of other recipes/targets, which is the case here (.build/_static/style.css is a side-effect of running Sphinx while executing the recipe for $(STAMP_SPHINX_COMPLETED))
    • Does not work as intended. Instead, grouped targets might be used. But probably not worth the hassle. I'm inclined to skipping this!
• remove obsolete files in the build:
  • .build/genindex/index.html
  • .build/objects.inv
  • .build/search/index.html
  • .build/searchindex.js
  • .build/_static/pygments.css
  • .build/_static/style.css.map
  • check for hidden files within .build/, e.g. .doctrees/ and .buildinfo
  • while running in CI, Sphinx's linkcheck builder is used. Probably it is creating files output.json and output.txt in the .build/ directory
    • verify!
    • rm them!