Docs: add llms.txt and robots.txt for AI assistants and answer engines

[tuhaihe]

Add root-level files to improve how AI assistants, answer engines, and search crawlers consume the site (GEO).

• llms.txt: follows the llmstxt.org convention. Concise project introduction (PostgreSQL kernel, Greenplum alternative, migration via cbcopy), key features, links to stable 2.x documentation, Greenplum migration guides, and community/GitHub resources.
• robots.txt: allows all crawlers and explicitly welcomes major AI crawlers (GPTBot, ClaudeBot, PerplexityBot, Google-Extended, and others), grouped by vendor, with a Sitemap reference.

Both files are served from the site root (/llms.txt and /robots.txt).

closes: #ISSUE_Number

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