docs: fix typos and rm unneeded sub-menu

[sympatheticmoose]

Noticed a few typos so tried to address, also seemed that the Hardware Engagement page didn't really require to be a section as just requires an additional click from the user.

• https://github.com/sustainable-computing-io/kepler-doc/blob/main/docs/design/metrics.md?plain=1#L68 This sentence generally doesn't read right to me with the mixed capitalisation on GPU etc, processeses definitely isn't correct, but not sure how to address.

I don't have mkdocs running locally, installing the requirements failed for me 🤷 so these changes would need to be verified (apologies)

[rootfs]

@sympatheticmoose thanks for having this!

@husky-parul @SamYuan1990 should we have a spell checker like this? https://github.com/marketplace/actions/github-spellcheck