Add new content and tidy up nesting of bullet points.
Add overview text and remove workflow section.
Reorganise sections and fix a few other issues.
merge develop -D org -I include -E exclude: PR 369 (Cpp: Correct content and markup)
merge develop -D org -I include -E exclude: PR 445 (Homebrew and Python (rebased onto develop))
merge develop -D org -I include -E exclude: PR 448 (Managed repository)