_content: fix markdown styling with dark theme under /src

This adds a template for markdown files under the go.dev/src path

Font color is set by a css variable under an Article selector.
The previous implementation did apply the Article class to pages
under the /src path.

By creating a default template pages will respect theme font colors.

Fixes golang/go#64929

Change-Id: Icc8e1866056553eaa9b2c2569e2c34cb9fb7fda5
GitHub-Last-Rev: f20f3c8aa5
GitHub-Pull-Request: golang/website#269
Reviewed-on: https://go-review.googlesource.com/c/website/+/553957
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

_content/src/default.tmpl

@@ -0,0 +1,3 @@
+{{define "layout"}}
+{{doclayout .}}
+{{end}}