Add changelog section to the README builder

[saurabhhhcodes]

Closes #199, #205, #209, #210, #212, and #215.

Adds a dedicated Changelog section to the README builder, improves icon-only button accessibility, makes the screenshot upload drop zone keyboard-operable, adds validation for the remaining URL fields, and now includes per-section reset controls so users can clear one section without wiping the whole form. It also fixes the missing section number gap in the editor. This update includes:

• a sidebar toggle and editor field for changelog entries
• template samples for common project types
• Markdown table output plus a raw collapsible details block
• persisted state across saves
• aria-label/title coverage for icon-only controls such as theme toggles, zoom controls, screenshot removal, and back-to-top
• keyboard activation for screenshot upload via Enter and Space
• validation warnings for Paper Link, Dataset Access, and API Base URL
• sequential editor section numbering
• per-section reset controls with a dedicated reset action in each editor header

This keeps changelog content consistent and makes the builder more accessible and polished.

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[saurabhhhcodes]

Hi, this PR closes #205 and #210. If it fits your scoring flow, could you please add the label along with one difficulty label and relevant type labels such as / ? Thanks!

[saurabhhhcodes]

Hi, this PR closes #205 and #210. If it fits your scoring flow, could you please add gssoc:approved plus one difficulty label and the relevant type labels for a feature/docs change?