Update FAQ to remove references to legacy code

[LordOfPolls]

About this pull request

auto-register and auto-delete were removed. This PR updates the FAQ to reflect this

Changes

Updated Add a slash command on Discord's example to use sync_commands not auto_register

Updated How to delete slash commands? to remove references to auto_delete

Checklist

• I've checked this pull request runs on Python 3.6.X.
• This fixes something in Issues.
  • Issue:
• This adds something new.
• There is/are breaking change(s).
• (If required) Relavent documentation has been updated/added.
• This is not a code change. (README, docs, etc.)

[i0bs]

Pending merge of this PR from eunwoo, will add this to the new docs that are under works of being re-designed.

[eunwoo1104]

Merging this pull request, thank you for contributing!