Fix HTML structure, broken links, typos, and accessibility across site

[jayvenn21]

Summary

Addresses multiple small issues found while reviewing the website repository.

HTML structure:

• Fixed orphan </article> tag in page layout (replaced with </div>)
• Removed stray </article> from lazy-consensus page
• Moved scripts before </body> in default layout
• Removed leftover standalone scripts and duplicate closing tags from collaborations page

Broken / outdated links:

• Fixed PHP gateway GitHub URL typo (= → -)
• Replaced deprecated git-wip-us.apache.org with gitbox.apache.org
• Removed dead DocumentUp link from learning page
• Fixed trailing whitespace in collaborations URL
• Upgraded HTTP links to HTTPS across development and mailing list pages

Typos:

• Fixed commmits → commits in mailing list unsubscribe mailto
• Fixed Septemeber → September (x2)

Accessibility / SEO:

• Added alt attributes to key images (layout, footer, homepage, about, get-involved, gsoc, learning)
• Added meta description tag
• Made page titles dynamic (per-page title | Apache Airavata)

Note:
There are further minor HTML changes that can be made including, but not limited to, misspelling and elements, but I have personally decided to scope this PR solely on the important fixes.

links to AIRAVATA-3971