[feat] Add base support of td-content-after-header hook

[chalin]

Important

This preliminary core feature implementation is experimental.

---

• Adds a "render hook" for page content right after the td-content <header>, and so just before the page .Content.
• Adds nothing more so that projects can experiment with use of site/page params etc. Docsy isn't ready yet to commit to richer solutions for Add a feature to warn readers that a blog post may be outdated. #1773 and the like.
• The base hook template is empty.
• Processes the template using .Render, which means that template lookup is based on content type.