Copy edits to Polaris contribution documentation

[jjgali]

Issue summary

As raised by @dGoligorsky in discussion #6762, there are a few copy edits to be made to the Polaris contribution documentation.

Expected behavior

• Copy edit: Figuring out what and when to contribute is not always simple, but it doesn't have to be! This reads like it doesn't have to be simple! Maybe could be clearer with something like [...] not always simple, so we're here to help. Or It can be tricky to know when to contribute. Here are some common scenarios [...]
• In the pledge, maybe disability could be ability ? (Regardless of [...] ability [...])?
• Little thing— in numbered lists (like on the contribute > Components page) sometimes there's a . at the end of the list item, sometimes not.
• Missing link in markdown on the Designing with a System page: If there's a gap, then [contribute](link to top of contribution page) to the system to make it better.
• Maybe put a link to the When to Contribute New Patterns guide in the Component section about adding a new pattern?
• Missing links in markdown of When to Contribute New Patterns guide: The key here is to [zoom out](link to thinking in systems section). and it's key to [contribute back to the system](insert link to contribution page) if[...]