Add Est. 2015 with link to first commit in footer

[Copilot]

Description

Adds "Est. 2015" to the footer that links to the repository's first commit (April 17, 2015), celebrating over a decade of TestingConferences.org.

Changes

• Modified _includes/footer.html to append | <a href="...">Est. 2015</a> to the copyright line
• Links to commit 1b028cfba42af75900682818914203c2f87532a4 (initial commit from April 2015)

Before:

© 2025 Software Testing Conferences and Workshops

After:

© 2025 Software Testing Conferences and Workshops | <a href="https://github.com/TestingConferences/testingconferences.github.io/commit/1b028cfba42af75900682818914203c2f87532a4">Est. 2015</a>

Additional context

Single line change. The "Est. 2015" link uses the site's existing link styling (brand green color).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Established Date</issue_title>
<issue_description>Now that we've passed a full decade of existing, I want to add an established date somewhere in the footer. If you click the "Est. 2015" it should link to the first commit. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Established Date #818

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