Add Markdown index HTML output mode

docs/config.md

@@ -54,7 +54,7 @@ pipelines:
 
 ## HTML Publication
 
-To publish generated HTML from Markdown files:
+To publish generated sidecar HTML from Markdown files:
 
 ```yaml
 publish:
@@ -68,6 +68,21 @@ transform:
 
 Sidecar generation writes `report.html` for `report.md`. It does not mutate the source bundle.
 
+To publish a single Markdown file as `index.html`:
+
+```yaml
+publish:
+  source: false
+  html: true
+transform:
+  markdown_to_html:
+    enabled: true
+    mode: index
+    input: report.md
+```
+
+When `mode: index` omits `input`, the source manifest must list exactly one Markdown file.
+
 ## Reference
 
 Top level:
@@ -164,7 +179,15 @@ Publish policy:
 - `publish.source`: publish source artifacts.
 - `publish.html`: publish generated HTML artifacts from Markdown source files.
 
-At least one output type must be enabled. When `publish.html` is true, `transform.markdown_to_html.enabled` must be `true` and `transform.markdown_to_html.mode` must be `sidecar`.
+At least one output type must be enabled. When `publish.html` is true, `transform.markdown_to_html.enabled` must be `true`.
+
+Markdown-to-HTML transform:
+
+- `transform.markdown_to_html.enabled`: enables Markdown-to-HTML generation for destinations with `publish.html: true`.
+- `transform.markdown_to_html.mode`: optional; defaults to `sidecar`. Accepted values are `sidecar` and `index`.
+- `transform.markdown_to_html.input`: optional source manifest path for `index` mode. It must identify a listed Markdown file.
+
+`sidecar` mode renders each manifest-listed `.md` file to a same-directory `.html` output. `index` mode renders one selected Markdown file to `index.html` at the destination bundle path. Enabled Markdown-to-HTML config is rejected when `publish.html` is false, and `input` is valid only with `mode: index`.
 
 Transfer policy:
 
@@ -184,6 +207,7 @@ Defaults are applied after YAML decoding and before validation:
 - SSH `host_key_policy: accept-new`
 - S3 `region: us-east-1`
 - S3 `force_path_style: true`
+- `transform.markdown_to_html.mode: sidecar` when a Markdown-to-HTML transform block is present and mode is omitted
 - `publish.source: true`
 - `publish.html: false`
 - `transfer.on_destination_same: skip`
@@ -216,6 +240,7 @@ Maintained examples live under [examples](../examples/):
 - `local-to-local.yml`: minimal local config.
 - `local-publish.yml`: runnable local source publication.
 - `local-html.yml`: runnable local HTML publication.
+- `local-index.yml`: runnable local `index.html` publication.
 - `fan-out.yml`: runnable local fan-out publication to source and HTML destinations.
 - `ssh-destination.yml`: environment-gated local-to-SSH publication example.
 - `s3-destination.yml`: environment-gated local-to-S3 publication example.