Small cleanup and additions to gender pronouns page.

Why is this change being made?

I just found out we had a page dedicated to Gender and sexual orientation identities. There were a number of things I felt I could improve on after reading through.

• Fixes some capitalization issues.
• Removes a reference to "above" when it looks like the referenced content was actually below (it was ambiguous so I just reworded it).
• Makes HRC a link the first time it appears in the document to avoid confusion.
• Adds /ace to Asexual as well as a new entry for Aromantic/aro.
• Avoids using the problematic verbiage "opposite sex" and "biological sex".
• Fixes a heading that I think should have been #### rather than ###.
• Fixes the headings for FAQ entries.
• Removes a potentially accidentally left over TODO comment.
• Adds a mention of singular they in addition to ze/hir/hirs.
• Includes a bit more detail/clarity on the FAQ for sex vs gender.
• Utilizes block quotes for the large quotations.