This PR wires up MkDocs’ edit URLs and shows a small “Edit this page on GitHub” link on docs and blog posts.

What changed
issue: #265

• Set repo_url and edit_uri in mkdocs.yml so MkDocs exposes page.edit_url.
• In theme/base.html, render a short “Found an issue? Edit this page on GitHub ↗” helper below the main content when page.edit_url is available.
• Added a tiny bit of CSS in theme/css/content.css so the link looks consistent with the rest of the theme.

Notes

• The helper lives in base.html, so it applies both to regular docs and to blog posts that use blog-post.html / other inherited layouts.
• The link always points to this repo (OpenScienceLabs/opensciencelabs.github.io) on the main branch.