In order to spell check, you just need to run this from anywhere in developer-docs-site/:
pnpm spellcheck
If you want to add a new word to the dictionary, you should add the word to additional_dict.txt and then run sort -o additional_dict.txt additional_dict.txt.
Note that aspell, at least as it is configured now, does not support adding words with numbers in them, such as MultiEd25519. Instead you'd just want to add MultiEd.