dx: add "Edit this page on GitHub" links for docs + blog pages

[Shivampal157]

Summary
The site is intended to be community‑driven, but there is currently no direct “Edit this page on GitHub” link on pages and blog posts. Adding this link would make it much easier for newcomers to propose fixes (typos, small content improvements, translations, etc.).

Motivation

• Lower the friction for first‑time contributors who notice a typo or outdated information.
• Align with OSL’s mission of teaching and mentoring people into open‑source workflows.
• This is a common pattern in MkDocs / documentation sites and will feel familiar to contributors.

Proposed changes

• Configure MkDocs to expose an “edit URL” for each page, pointing to the corresponding file in OpenScienceLabs/opensciencelabs.github.io on GitHub.
• Add an “Edit this page on GitHub” link in the custom theme (e.g. in the page footer or near the title) for:
  - Regular documentation pages under pages/
  - Blog posts under pages/blog/**/index.md
• Ensure the link respects the current branch (main) and file paths used in this repo.

Implementation ideas

• Use the MkDocs edit_uri / repo_url configuration in mkdocs.yml to generate edit links.
• Update the relevant Jinja templates in theme/ (e.g. main.html, blog-post.html) to render a small call‑to‑action like:
  - “Found an issue? Edit this page on GitHub ↗”
• Optionally add a small icon and style it using the existing theme CSS.

Acceptance criteria

• Every documentation page and blog post shows a working “Edit this page on GitHub” link.
• Clicking the link opens the correct file in the GitHub web editor for this repo.
• The link is unobtrusive but visible (e.g. near the footer or page title).

[Shivampal157]

Hi @xmnlab I have created this issue.
Could you please review it and assign it to me if it looks good?