Further polish CONTRIBUTING.rst
Further polish CONTRIBUTING.rst

- Update the language/wording
- Fix several typos and improve grammar
- Fix brokend links and add new ones
- Replace single ` with double `` for code pieces in paragragh

felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer,felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer,felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer,CenterForOpenScience/modular-file-renderer,felliott/modular-file-renderer