Add Earn Bitcoin page

[KingParmenides]

Summary

Adds an English Earn Bitcoin page for bounty #2524.

Changes:

• adds a new earn page with practical ways to earn bitcoin through work, selling, creating, mining, and open source contributions
• adds an English-only Participate menu/footer link so untranslated pages do not get links to pages that are not generated
• omits the homepage button, matching the maintainer feedback in the issue discussion
• includes a safety section warning against upfront-deposit, private-key, and easy-return scams

I checked the external links before opening the PR and avoided links that failed or redirected to unrelated pages.

Bounty

Addresses #2524.

BTC bounty payment address: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ

Testing

• git diff --check -- _templates/earn.html _includes/layout/base/head-menu.html _includes/layout/base/footer-menu.html _translations/en.yml
• ruby -e "require 'yaml'; YAML.load_file('_translations/en.yml'); puts 'en.yml ok'"
• checked the earn translation keys, earn URL key, and menu-earn layout key are present
• checked every icon referenced by _templates/earn.html has both .svg and .png assets
• checked the external links used on the page returned final HTTP 200 responses

Full local Jekyll build was not run because this workspace has Ruby 2.6.10 while the repository pins Ruby 2.5.8 in the Gemfile; a verification-only bundle attempt was also blocked by native ffi build failure on this macOS arm64 environment.