Keep improving CI

[BryanQuigley]

this includes spellcheck for both other files and markdown
updated lint script that has more options - including formatting
enabled markdown link checker as the first pass on making sure links work
update CI to only activate on changed files

[lydiascarf]

lgtm! i like the linter running only on changes by default, and it ran as expected when i tested it.

i tested against odp as well and it was mostly good! the only thing was we have this line in our megalinter config:

BASH_SHFMT_ARGUMENTS: "-i 4"

and cspell didn't like "SHFMT". this env is telling the bash linter to use 4 spaces instead of tabs, which i like. maybe we should add that in here as well and address the cspell error before merging