Limit deploy previews to mounted docs roots

[jdbaldry]

What

• Keep deploy preview docs navigation scoped to mounted content only.
• Derive mounted top-level docs roots from SOURCES and remove other /hugo/content/docs/* trees before Hugo build.
• Add title and menuTitle to generated redirect index pages so root sections have labels.

Testing

• Validated against grafana/mcp-doc-server PR 36 with toolkit_ref set to jdb/deploy-preview.
• Before: sidebar included unrelated product trees and blank labels.
• After: sidebar contains only the mounted mcp-doc-server tree and zero empty docs menu anchors.

[github-actions]

Readability report

File	AutomatedReadability	ColemanLiau	FleschKincaid	FleschReadingEase	GunningFog	LIX	SMOG
docs/sources/whats-new.md	$${9.69\space\color{black}(Δ+0.00)}$$	$${13.09\space\color{black}(Δ+0.00)}$$	$${9.74\space\color{black}(Δ+0.00)}$$	$${46.61\space\color{black}(Δ+0.00)}$$	$${11.47\space\color{black}(Δ+0.00)}$$	$${70.33\space\color{black}(Δ+0.00)}$$	$${11.21\space\color{black}(Δ+0.00)}$$

View metric targets

Metric	Range	Ideal score
AutomatedReadability	6 (very easy read) to 14 (extremely difficult read)	8 or less
ColemanLiau	6 (very easy read) to 17 (extremely difficult read)	9 or less
FleschKincaid	6 (very easy read) to 17 (extremely difficult read)	8 or less
FleschReadingEase	100 (very easy read) to 0 (extremely difficult read)	70 or more
GunningFog	6 (very easy read) to 17 (extremely difficult read)	10 or less
LIX	20 (very easy read) to 60+ (extremely difficult read)	35 or less
SMOG	6 (very easy read) to 17 (extremely difficult read)	10 or less

[github-actions]

💻 Deploy preview available (Serve redirects in deploy previews from the Hugo manifest):

• https://deploy-preview-writers-toolkit-1477-zb444pucvq-vp.a.run.app/docs/writers-toolkit/