Guides top navigation like Rails homepage

[p8]

This makes the top navigation of guides more similar to the Rails homepage by using the same fonts, styling and logo as the Rails homepage.

The "home" and "Contribute" links in the secondary navigation are removed, as these are directly visible in the top navigation.

Before

After

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[zzak]

Just a thought, but in the mobile screen it's hard to tell the logo is a button to open the menu IMO. I know this came up in the splash screen too and they added a drop shadow. I'm still not convinced though 🤔

[zzak]

I'd like to hear some other opinions about the "logo vs. button" thing, but another thought I had was just about the "Guides Index" feeling too long, maybe that can just always be a hamburger menu after we move the other links out?

Ideally the top navigation should be consistent across the website, api docs, and guides IMO. 🤔