Recommend bookdown-appendix approach

[crsh]

• [x] Recommend the manual placement of bibliography and use of special appendix header
• [x] # (APPENDIX) Appendices {-}, but not if you omit the {-}
• [x] Appendices must start with an (empty) H1
• [x] Note that the YAML front matter option appendix still exists but mainly for backward compatibility

[crsh]

Also see https://github.com/crsh/papaja/issues/526