docs: refresh README for Modrinth

[Jakubk15]

Summary

• rewrite the README around the plugin's player-facing delivery experience
• document verified features such as parcel sizes, priority delivery, returns, economy fees, administration tools, and optional Discord notifications
• add a Modrinth-focused quick start, requirements, project status, and download call-to-action
• preserve contributor, support, development-build, and sponsor information

Why

The README is synchronized directly to Modrinth, the project's main publishing platform. The previous copy was sparse, contained discouraging early-stage wording, and did not explain why server owners or players should try ParcelLockers.

Impact

Server owners now get a scannable overview of compatibility, setup, permissions, customization, and integrations, while players get a clearer picture of the gameplay experience the plugin adds.

Validation

• ./gradlew.bat test — BUILD SUCCESSFUL
• git diff --check — clean
• verified required Modrinth, Paper/Purpur, Java 21, Vault, DiscordSRV, parcel return, priority-delivery, and permission content
• verified no relative README image URLs or unsupported Folia/Nexo claims

[gemini-code-assist]

Code Review

This pull request updates the project's documentation by rewriting the README.md to be more engaging, benefit-led, and optimized for Modrinth. It also adds a design specification and an implementation plan under the docs directory. The reviewer feedback suggests replacing indirect GitHub blob URLs with direct raw.githubusercontent.com URLs across both the README.md and the implementation plan to avoid HTTP redirects and ensure reliable rendering on external platforms.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[gemini-code-assist]

Using direct raw.githubusercontent.com URLs is preferred over github.com/.../blob/.../?raw=true because it avoids an HTTP redirect and is more reliable when rendered on external platforms like Modrinth.

Suggested change

	[<img src="https://github.com/EternalCodeTeam/ParcelLockers/blob/master/.github/assets/sentry.svg?raw=true" alt="Sentry logo" width="150" height="150">](https://www.sentry.io)
	[<img src="https://raw.githubusercontent.com/EternalCodeTeam/ParcelLockers/master/.github/assets/sentry.svg" alt="Sentry logo" width="150" height="150">](https://www.sentry.io)