x/website: revamp website for precise and pointed downloads, better access with content indices + user experience

[odeke-em]

We are revamping the website for https://go.dev and to that effect we have a couple of tasks whereby the finished prototype is at https://golang-doc-6yhxy5np6a-uc.a.run.app/ and to get changes in smoothly per orijtech/website#1 and this is our game plan. This issue is merely a progress tracker

Tasks

• Commit css changes: _content/css/* updates, Javascript changes _content/js
• Commit _content/doc/tutorial/<EDITOR_GUIDES.md>, Gopls, VimGo, VSCode and the various images
• Implement primary/left/title index
• Implement secondary/right/in-page index
• Implement the in-page progress jump buttons
• Implement new download experience
  • Content page firstly for where the downloads will be redirected to
  • UserAgent detection and redirection
  • Lastly the download button plus "Tour Go"
• Update all the menus accordingly

/cc @willpoint @jamalc @joedian

[gopherbot]

Change https://go.dev/cl/532055 mentions this issue: _content/css: add docs.css

[gopherbot]

Change https://go.dev/cl/540115 mentions this issue: _content: include bases for website v2

[gopherbot]

Change https://go.dev/cl/540696 mentions this issue: _internal: add tests per navigation feature included in site.tmpl

[gopherbot]

Change https://go.dev/cl/541855 mentions this issue: _content: new download button, tutorials + grammar updates

[gopherbot]

Change https://go.dev/cl/543235 mentions this issue: _content: add new download button

[gopherbot]

Change https://go.dev/cl/543077 mentions this issue: _content: bring in updated images/icons

[gopherbot]

Change https://go.dev/cl/543295 mentions this issue: _content: bring in css/docs.css

[gopherbot]

Change https://go.dev/cl/543078 mentions this issue: _content: bring in docs.yaml

[gopherbot]

Change https://go.dev/cl/543079 mentions this issue: _content, internal/web: bring in site rendering updates

[gopherbot]

Change https://go.dev/cl/543080 mentions this issue: _content/js: bring in docs.js in website revamp