Auto-detect `siteUrl`

[shivaraj-bh]

Manual override of the og:image requires a page.image metadata in the README, like:

---
page:
  image: <my-image.png>
---

# Foo Blog
...

But this doesn’t work if the siteUrl is not configured in index.yaml. See for example:

Before:

After including siteUrl in index.yaml:

[srid]

https://github.com/juspay/services-flake/commit/6533cd23526ba1a73286b5fe8108c2f5ff370253 solves it for services-flake, but not other modules.

A general solution would involve configuring siteUrl automatically ... something to consider as part of https://github.com/srid/emanote/issues/494