Significant docs cleanup and refresh

[robscott]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
This PR is pretty massive, here's an attempt at a changelog:

Logo and Design

• Logo files have been added
• Logo replaces title in header
• Logo added to main page
• Header background/overall primary color changes to Kubernetes primary color
• Updated main diagram on front page and added link to file that can be used to edit

Mesh

• A new "Mesh" section has been added, including Overview, GAMMA, and Service Facets Pages. (None of this content is new, it's just all more neatly contained in a Mesh-specific section).
• Once this merges, gateway-api.sigs.k8s.io/mesh will get you to this section
• Redirects from old pages have been added

Layout

• Tabs now show up on small displays (previously they'd hide below ~1200px width).
• GEPs have been moved from "reference" to a new "enhancements" tab
• Blog tab/navigation has been hidden, but individual pages remain for the sake of any old links out there
• Sidebar navigation is now fully expanded by default (not sure about this one, but it mostly feels like an improvement)

Docs Consistency

• A new documentation style guide has been added to extend the Kubernetes upstream one
• Project name has been clarified as "Gateway API", references to "the Gateway API" have been corrected
• "Experimental" and "Standard" channel callouts have been standardized
• Absolute links have been replaced with relative ones wherever possible

Misc

• Style of FAQs has been cleaned up throughout, now questions are included in navigation

Which issue(s) this PR fixes:
Fixes #2553

Does this PR introduce a user-facing change?:

NONE

[robscott]

Recommend spending some time looking at the deploy preview when reviewing this one: https://deploy-preview-2691--kubernetes-sigs-gateway-api.netlify.app/.

Adding a hold since this is a big PR and want to give people a chance to look through it.

/hold

[youngnick]

Nice, this LGTM (as long as it merges after #2689 and I don't have to rebase 😄 )

[youngnick]

LGTM aside from one thing, so I'll leave this for another round of changes, sorry @robscott.

[youngnick]

Does it make more sense to have this section sorted in roughly declining order of stability? I'm thinking:

1. Memorandum
2. Standard
3. Experimental
4. Implementable
5. Provisional

[robscott]

Since this PR is big enough to conflict with ~everything docs related, I'm going to merge as is and make a follow up for this detail so I don't have to do any more rebases.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: robscott, youngnick

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[shaneutt]

/lgtm

[robscott]

Will make a follow up PR to address @youngnick's final comment, but getting this in now to avoid more rebasing.

/hold cancel