A number of (much needed) updates and corrections to the
dev-model book:

• Make pronouns truly gender-agnostic
• Correct typos and minor grammar errors
• Update number of ports vs. time graph to a less outdated version

(from [1])

• Enhance screenreader friendliness of pictures
• Remove reference to deleted image
• Add link to FDP primer

Thanks to Pau Amma for working on a patch and submitting it
to us.

[1] https://web.archive.org/web/20121223081637if_/http://www.freebsd.org/ports/growth/status.png

PR: 241814
Submitted by: pauamma_gundo.com
Reviewed by: debdrub, bcr, allanjude
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D24321