Use vale to systematically improve the quality of pages

[cristiklein]

Let's use vale.sh to reduce the probability of introducing spelling and grammar mistakes. Vale should be triggered at three levels:

1. Local development, via vim or VSCode integration (see VIM ALE.)
2. Pre-commit
3. CI at PR creation time (likely by running pre-commit run --all via GitHub Actions).

[cristiklein]

The terms-internal project could serve as inspiration for implementation.