Playbook: branding update 2nd pass

[drew-usds]

Some basic branding updates were made in playbook/231, but let's do another pass!

• [ ] Re-work the intro section to use the standard, non-seal logo. Presently is using the seal logo + a navy backdrop to preserve some of the original design and address a possible readability concern with the body copy.
• [ ] Change sticky nav numerals to sans to match the play # titles
• [ ] Edit font size or styling on play # titles, currently could be hard to read.
• [ ] Do we need the /techfar page? It seems like hanging content. The only internal mention links directly to techfarhub.cio.gov. Does it link from anywhere outside of the playbook?
• [ ] Do a quick responsive/mobile audit to see if there's anything to improve.