docs: follow-ups to builders page refactor

[maggieneterval]

Follow-up to: #8382

Description

Follow-ups to #8426 discussed offline:

• Unnest all the child nodes of the Skaffold Pipeline Stages nav item, so that they are now top-level nav items.
• Unnest the Cross/multi-platform item so that it is a top-level child of the Build nav item.
• Update all aliases for proper URL redirection, and verify that this is working as expected.
• Add links to environments axis of builders x environments table.

User facing changes

Before:

After:

[codecov]

Codecov Report

Merging #8449 (74dc5f8) into main (290280e) will decrease coverage by 5.27%.
The diff coverage is 55.02%.

@@            Coverage Diff             @@
##             main    #8449      +/-   ##
==========================================
- Coverage   70.48%   65.22%   -5.27%     
==========================================
  Files         515      603      +88     
  Lines       23150    29898    +6748     
==========================================
+ Hits        16317    19500    +3183     
- Misses       5776     8923    +3147     
- Partials     1057     1475     +418     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/completion.go	13.04% <0.00%> (-1.25%)	⬇️
cmd/skaffold/app/cmd/config/list.go	65.21% <ø> (ø)
cmd/skaffold/app/cmd/config/set.go	88.72% <ø> (ø)
cmd/skaffold/app/cmd/config/util.go	54.28% <ø> (ø)
cmd/skaffold/app/cmd/credits.go	100.00% <ø> (ø)
cmd/skaffold/app/cmd/credits/export.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/deploy.go	40.90% <0.00%> (-12.94%)	⬇️
cmd/skaffold/app/cmd/generate_pipeline.go	60.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_modules.go	65.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_profiles.go	66.66% <ø> (ø)
... and 425 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[aaron-prindle]

Awesome, just loaded it up locally, looks great! LGTM!