feat: add edit this page on github link to every page footer

[trysky0318-lang]

What does this PR do?

Adds an "Edit this page on GitHub" link to the footer of all seven pages. Each link points at that page's own file on main, so a typo can be fixed in a few clicks.

Linked issue

Closes #43

How did you test this?

• Served the site locally with python -m http.server 8000 and opened index.html: the link appears in the footer's "Contribute" column, styled like the other footer links.
• Toggled the dark theme with the header button: the link looks correct in both themes. No new CSS was added; it reuses the existing footer link styles.
• Checked that all seven target URLs (index, faq, git-basics, glossary, pr-checklist, wall, 404) exist on main and return HTTP 200.
• Ran the same internal-link check as CI's links job locally: every local link resolves.
• Not yet checked at 375px wide; the footer layout is unchanged, so I expect no difference.

Screenshots / terminal output

Footer screenshots (light and dark themes) are attached in a comment below.

Checklist

• I claimed this issue and it is assigned to me
• My branch is named feat/..., fix/..., or docs/...
• I only changed files related to this one issue
• I ran the project locally and it works
• CI checks below are green (or I have said why they are not)
• I am not adding my name to the README or reformatting files just to raise a PR

[github-actions]

Congratulations on your first pull request to this repository.

Here is what happens next:

1. The checks below need to go green. If one is red, click Details to see why — that is normal, and fixing it is part of the work
2. A maintainer reviews within 48 hours and may leave line comments
3. Once it is approved, a maintainer merges it

Two things that get PRs closed, so please double-check:

• No Closes #<issue> in the description
• The issue was never assigned to you

Nice work getting this far. Most people never open their first PR.

[trysky0318-lang]