docs: rewrite Model Providers page and finish 1.15.0 terminology cleanup

[RiskeyL]

Summary

• Rewrite the Model Providers page (en/zh/ja). Trimmed to the tasks the UI doesn't make self-evident, with the navigation and button labels (Integrations > Model Provider, Setup/Configure, Add Model, Manage Credentials, Default Models) verified against the current 1.15.0 i18n. Adds the install-from-Marketplace step and a Default Models section, explains when you add a model manually versus when models are ready to use, and drops the stale Supported Providers list, frozen config examples, UI-restating permission list, and shallow troubleshooting.
• Home page: the Marketplace card now says "integrations" instead of "plugins" to match the product. The Develop Plugins card stays, since the develop-plugin tree keeps "plugin" for developers.
• TRIGGER_URL env var: renamed "plugin trigger" to "integration trigger" in the description (en/zh/ja), the one spot the navigation pass didn't reach.
• Formatting guide: relaxed the em-dash rule from a hard one-per-paragraph cap to a judgment-based standard (use sparingly, don't stack or pile up, but not banned).

Chinese and Japanese are synced across all four changes.

[Copilot]

Pull request overview

This PR updates Dify documentation to match 1.15.0 UI terminology and guidance, centered on a rewritten “Model Providers” page and a terminology cleanup from “plugin” to “integration” where applicable.

Changes:

• Rewrites the Model Providers page (en/zh/ja) to focus on Marketplace installation, adding providers, adding custom models, managing credentials, and setting default models.
• Updates the home page Marketplace card copy (en/zh/ja) from “plugins” to “integrations”.
• Renames “plugin trigger” to “integration trigger” in TRIGGER_URL environment variable documentation (en/zh/ja) and relaxes the em-dash guidance in the formatting guide.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
en/self-host/use-dify/workspace/model-providers.mdx	Rewrites the page structure and instructions; adds Marketplace + Default Models guidance.
zh/self-host/use-dify/workspace/model-providers.mdx	Chinese version of the rewritten Model Providers page.
ja/self-host/use-dify/workspace/model-providers.mdx	Japanese version of the rewritten Model Providers page.
en/self-host/deploy/configuration/environments.mdx	Updates TRIGGER_URL description from “plugin trigger” to “integration trigger”.
zh/self-host/deploy/configuration/environments.mdx	Chinese TRIGGER_URL terminology update.
ja/self-host/deploy/configuration/environments.mdx	Japanese TRIGGER_URL terminology update.
en/home.mdx	Updates Marketplace card copy to say “integrations”.
zh/home.mdx	Chinese Marketplace card terminology update.
ja/home.mdx	Japanese Marketplace card terminology update.
writing-guides/formatting-guide.md	Updates em-dash guidance to a judgment-based standard and adjusts the checklist.

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