fix grammar errors + toss .DS_Store into the shadow realm + markdown lint fixes

Hello all and especially @sabrina-aip! Just a few minor fixes that include:

Correcting Sabrina's typo of the word 'spelling' as in 'spelling bees' in the README (how ironic)
Point towards the actual video in the README
Banishing .DS_Store and making sure that it never appears again
Calming down my markdown linter (https://marketplace.visualstudio.com/items?itemName=DavidAnson.vscode-markdownlint)
Other miscellaneous grammar fixes

sabrina-aip / spellcheck