docs: improve FAQ

[killme2008]

What's Changed in this PR

Describe the change in this PR

As the title said.

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

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[Copilot]

Pull Request Overview

This PR improves the FAQ documentation by restructuring and expanding content to provide more comprehensive guidance about GreptimeDB's capabilities, deployment options, and use cases. The update transforms a basic Q&A format into a detailed guide covering core capabilities, architecture, integrations, and best practices.

Key changes:

• Complete restructuring of FAQ content with organized sections (Core Capabilities, Architecture & Performance, Integration & Compatibility, etc.)
• Addition of detailed performance comparisons with specific metrics and benchmark data
• Enhanced technical details about GreptimeDB's architecture, storage engine, and scalability features
• Cross-references to FAQ from concept overview pages to improve discoverability

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
i18n/zh/docusaurus-plugin-content-docs/current/faq-and-others/faq.md	Complete rewrite of Chinese FAQ with comprehensive sections covering capabilities, performance, deployment, and technical details
docs/faq-and-others/faq.md	Complete rewrite of English FAQ with expanded content matching the Chinese version structure
i18n/zh/docusaurus-plugin-content-docs/current/user-guide/concepts/why-greptimedb.md	Updates WAL description to mention flexible options and removes planned Python feature
docs/user-guide/concepts/why-greptimedb.md	Same WAL description update as Chinese version
i18n/zh/docusaurus-plugin-content-docs/current/user-guide/concepts/overview.md	Adds FAQ link to concepts overview navigation
docs/user-guide/concepts/overview.md	Adds FAQ link to concepts overview navigation
i18n/zh/docusaurus-plugin-content-docs/current/user-guide/concepts/features-that-you-concern.md	Fixes typos in section headers and adds FAQ cross-reference
docs/user-guide/concepts/features-that-you-concern.md	Fixes typo in section header and adds FAQ cross-reference

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[killme2008]

After review, backport to v0.15 and v0.16. Don't merge it.

[nicecui]

LGTM 👍