docs: add changelog entry for updates since v0.6c

[P3rlE]

Motivation

• Provide a concise, user-facing summary of work done since v0.6c so players and packagers can see recent features and fixes.
• Group recent development into Added, Improved, and Fixed sections to make the changelog easier to scan.
• Surface UI, gameplay, racing-bot, graphics, mapping docs, and stability fixes that are visible to end users.

Description

• Inserted a new new since v0.6c section at the top of changelog.txt with grouped bullets for Added, Improved, and Fixed items.
• Summarised recent work such as the in-game HUD Elements menu (Shift+H), Content Manager download system, Advanced Graphics menu, hybrid racing waypoint system, staged func_breakable damage, mapping GamePack and guide updates, and controller/profile UX changes.
• Included stabilisation and UX fixes like skipping the GFX loading screen on main menu return, credits scrolling fixes, UI hitbox fixes, race-observer fallback crash fix, and derby damage CVar validation.
• This change is documentation-only and modifies only changelog.txt.

Testing

• Verified recent commits used for the summary with git log --oneline 2b418ed..HEAD, which completed successfully.
• Confirmed repository status after the edit with git status --short, which showed only changelog.txt as modified.
• Printed and reviewed the updated file with nl -ba changelog.txt | sed -n '1,120p', which showed the new new since v0.6c section as expected.

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Codex Task