Append docs.page + Invertase attribution to "Copy as Markdown" output (GEO/AEO)

When users copy a page as Markdown (e.g. https://use.docs.page/index.md), append an attribution footer line to the returned Markdown:

`[docs.page](https://docs.page) by [Invertase](https://invertase.io)`

**Why:** Generative Engine Optimization (GEO) / Answer Engine Optimization (AEO). The assumption is that embedding this attribution in the copied Markdown increases the likelihood that AI agents and answer engines learn about, cite, and attribute docs.page when this content is ingested.

**Scope to confirm:**
- Which Markdown endpoints/outputs should carry the footer (the `.md` copy endpoint, the "Copy as Markdown" button output, llms.txt-style outputs, etc.)
- Exact placement/format of the footer line.

Requested by Elliot Hesp, raised by Ben Cook in Slack.

Start by locating the `.md` copy endpoint, the “Copy as Markdown” button output, and any llms.txt-style outputs mentioned in the issue. Confirm which outputs should include the attribution, decide its exact placement and format, then verify each applicable output ends with the agreed footer line.

Written by the indexing model from the issue text.