docs: complete WebShopX documentation restructure

[Cc-Cece]

Summary

This PR completes the WebShopX documentation restructuring pass and turns the docs into a role-based, task-oriented structure with explicit reference/source-of-truth boundaries.

Information architecture

• Added a dedicated WebShopX Getting Started page.
• Reworked the WebShopX sidebar into:
  • Player Guide
  • Server Administrator
  • Developer
  • Reference
• Kept WebShopX-Payments as a separate product section while documenting its relationship to WebShopX core.
• Added a Reference index that points to the authoritative config, command, API, error, glossary, and migration pages instead of duplicating parameters.

Player documentation

• Rewrote Quick Start around the current v3 flow: /ws password -> /ws home -> web login.
• Removed stale registration-oriented guidance from the normal player path.
• Updated mailbox usage to /ws mailbox collect.
• Simplified the web-inventory page so players see online/read-only/experimental behavior without low-level playerdata implementation details.
• Added explicit Stable / Beta / Experimental semantics to the player guide.

Server administrator documentation

• Rebuilt configuration.md from the current WebShopX config.yml baseline.
• Removed draft-only/non-shipped config keys from the primary configuration example.
• Reworked installation/deployment around SQLite, MySQL/MariaDB, internal, external, and relay modes.
• Clarified the v3 admin entry behavior and bootstrap-account security requirements.
• Updated current commands and permissions and removed old v2 command guidance from the primary workflow.
• Fixed broken links in the administrator overview and aligned it with the actual page filenames.
• Normalized filenames to install-deploy.md and commands-permissions.md while preserving document IDs/routes.

Developer documentation

Promoted the former hidden .dev/dev/* material into public docs/webshopx/developer/* documentation:

• developer overview
• HTTP API basics
• authentication and sessions
• player API
• market API
• admin API
• errors and idempotency
• uploads and static assets
• integration playbook

The superseded hidden .dev/dev/* copies were removed so the public developer docs are now the maintenance source.

The existing WebShopXPaymentApi guide remains the Java/plugin-side Payment Provider integration entry.

English i18n

• Updated the English site landing page, player overview/quick start, administrator overview/configuration, install/deploy, and commands/permissions.
• Added English Getting Started, Developer Overview, Reference, core HTTP API pages, and web-inventory guidance.
• Migrated the English admin translation paths to match the normalized source paths and removed the old orphaned paths.

Source-of-truth policy introduced by this restructure

1. The WebShopX version's code / shipped configuration is authoritative.
2. Reference documentation summarizes that implementation.
3. Player/admin guides explain task flows without duplicating whole parameter sets.
4. README files, old screenshots, and third-party guides are secondary references.

Validation performed

• Compared the full branch against main; the branch is based directly on current main and is not behind it.
• Reviewed sidebar document IDs and normalized source/i18n paths.
• Checked the repository's frontmatter validation requirements; all newly added docs/webshopx/* pages include kebab-case IDs, titles, and sidebar positions.
• Cross-checked high-impact WebShopX facts against the connected Cc-Cece/WebShopX repository, including current config.yml, plugin.yml, deployment modes, and API implementation locations.
• Attempted to run the repository validation/build locally, but this execution environment cannot resolve github.com, so a fresh checkout/dependency install cannot be completed here.
• The current PR head has no reported GitHub commit-status checks, so final Docusaurus rendering/build validation still needs repository CI or a local preview.

Manual review requested

Please focus manual preview on:

1. WebShopX sidebar/category navigation (ZH and EN)
2. relative links from Getting Started / Developer / Reference pages
3. administrator install/config pages
4. player Quick Start and web-inventory pages
5. English route/path migration

The PR intentionally remains Draft until that visual/build review is completed.