docs: rewrite Workbench how-to guides to match current UI flow

[wardbryan3]

Summary

Rewrote the Workbench how-to guides section against the vantage-ui source
of truth. The previous guides described stale wizard steps, navigation paths,
and button labels that no longer matched the product.

Changes

Rewrote from scratch (were sparse or inaccurate)

• serve/deploying.mdx — old 7-step outline → current 3-step
  Deploy Inference Endpoint wizard (Identity, Model source, Tuning)
• train/submitting-a-job.mdx — old 6-step wizard → current
  Preset/Custom mode dialog with correct fields
• train/registering-a-model.mdx — 3-line placeholder → full guide
  covering HuggingFace, S3, URL, and local source types
• serve/autoscaling.mdx — outdated standalone guide → correctly
  describes autoscaling driven by inference presets
• serve/canary-rollouts.mdx — overstated promote/rollback claims →
  realistic read-only traffic-display description

Aligned navigation, labels, and fields with the current UI

• Navigation paths corrected throughout:
  Workbench > Develop, Workbench > Compute > Pools,
  Workbench > Train > Training > Runtimes, etc.
• Button labels match UI: New Compute Pool, Deploy endpoint,
  Launch job, New cloud shell, etc.
• Cloud Shell, Remote Desktop, PVC Viewer forms updated to reflect
  actual fields (Compute Profile, Slurm Cluster, Advanced Options)
• manage-services.mdx rewritten to describe the user-service
  lifecycle (Cloud Shell, Remote Desktop, PVC Viewer)
• create-preset.mdx updated: presets live under
  Compute > Presets, not a Training-only page
• configure-runtime.mdx updated to match current dialog
  (base image, ML policy fields per framework, compute pool)
• create-pool.mdx fields matched to current New Compute Pool
  dialog
• creating-a-sweep.mdx now documents the UI creation dialog
  alongside the SDK path

Formatting and consistency fixes

• observability.mdx rewritten with proper guide-intro pattern
• Sidebar labels in sidebars-main.js aligned with H1 headings
• Backtick-wrapped literal values in troubleshooting.mdx
• Removed duplicate/misleading link text in create-pool.mdx
• Removed orphan success-state line from troubleshooting page

Left unchanged (already accurate)

• creating-a-session.mdx
• creating-a-preset.mdx (session presets)
• managing-sessions.mdx
• Pipeline guides (pipelines/creating, running, scheduling)
• troubleshooting.mdx (aside from minor fixes above)

Testing

1. node scripts/fetch-v8x-cli-docs.js && yarn build — should succeed
2. yarn spellcheck docs/how-to-guides/workbench — only expected
   technical terms flagged (ttyd, CUDA, cuda, cudnn, rstudio,
   gaudi, millicores, etc.)

Notes

• The compute profiles vs compute pools terminology conflict
  predates this change and is tracked across the broader docs tree.
• The generated CLI docs (reference/developer-tools/cli/*) and
  yarn.lock appear in the diff from the build step but have no
  meaningful changes.