Draft: feat: add German spell checking

Issue: related to #293

This MR more or less follows https://github.com/streetsidesoftware/cspell-cli/blob/main/docker/german/README.md to add German to the spell checking capabilities (docs, cspell config, CI). On developer systems, manual intervention is necessary (i.e. to install the German dictionary) or the cspell config will fail so this is a somewhat breaking change.

It also specifies to use British English to eliminate ambiguity. This was chosen because Matrix is based in England.

This is a backport from the architecture repo (solution-architecture!13 (merged)) which despite aiming to be primarily in English sometimes needs to reference German words or excerpts.

It is to be decided whether this is actually useful to our wider project. I'm more confident that it will be useful for other projects deciding to use our template.