docs: grammar edits, sentence-case secondary headings

docs/Acceptable-Formulae.md

docs/Analytics.md

docs/Bottles.md

docs/Brew-Test-Bot-For-Core-Contributors.md

docs/Common-Issues.md

docs/Custom-GCC-and-cross-compilers.md

docs/External-Commands.md

docs/FAQ.md

docs/Formula-Cookbook.md

docs/Gems,-Eggs-and-Perl-Modules.md

docs/Homebrew-and-Python.md

docs/How-To-Open-a-Homebrew-Pull-Request.md

docs/How-to-Create-and-Maintain-a-Tap.md

...e-outside-Homebrew-with-Homebrew-keg-only-dependencies.md

docs/Installation.md

docs/Interesting-Taps-&-Forks.md

docs/Kickstarter-Supporters.md

docs/Maintainer-Guidelines.md

docs/Maintainers-Avoiding-Burnout.md

docs/Node-for-Formula-Authors.md