Refine 'gaining commit privileges'.

[jaraco]

During my recent experience nominating someone for commit privileges, I ran into some friction attempting to follow the guidance. This change addresses that friction.

• Allow the audience to be anybody (not just an individual seeking commit privileges). Avoids use of second-person language.
• Provide guidance for all involved.
• Refer to the various actors directly, avoiding passive language. Use 'candidate', 'submitter', and 'approver' to represent the key actors.
• Move the "cross-post" action into step 1 to indicate better the temporal positition of the action.
• Summarize the confirmation criterion (2/3 vote) rather than make the reader go to the PEP, which basically indicates 2/3 vote.
• Include the steering council email, because the submitter may not know it.
• Normalize the initial approver actions. Previously, council was directed to be "asking for ... a reminder about the Code of Conduct and to report issues to the PSF WG". Now it's two requests and a reminder.
• Hyperlink "python/voters" instead of the full URL.