[hotfix] Fix broken links and markup in website source

[spuru9]

What is the purpose of the change

Fixes six small defects in the website sources: invalid markup, links to a URL scheme retired in the 2023 redesign, a heading copied from the wrong file, and a subproject status that changed in 2024.

Brief change log

• footer.html: Security link closed with </a-->, emitting invalid markup in the footer of every page in both languages
• getting-started/_index.md (en, zh): heading rendered as "Documentation"; now matches each file's own frontmatter title
• content.zh/how-to-contribute/contribute-code.md: three links to /zh/contributing/*.html have 404'd since the 2023 redesign (FLINK-33046); replaced with the relref shortcodes the English page already uses
• downloads.md (en, zh): added /downloads.html aliases (~50 release blog posts link there), matching the pattern in community.md and roadmap.md
• what-is-flink-table-store.md (en, zh): described Paimon as incubating; it has been a top-level project since 2024-04-16
• content.zh/what-is-flink/community.md: removed the Paimon repo entry pointing at the retired incubator-paimon repos, as the English page did in 9d0761c (FLINK-34946)

The last item is the only judgment call; happy to repoint the URLs instead if preferred.

Verifying this change

Built with Hugo 0.124.1 extended and checked in the generated output: both aliases redirect, the two relref links resolve under /zh/how-to-contribute/, and no a--> or incubator-paimon references remain. Source files only; content/ left for the committer rebuild.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)
• Chinese pages here changed only mechanically; the remaining content.zh gaps need a native speaker and are headed for chinese-translation Jiras (FLINK-13680 already covers the code-style page)

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude Code (Claude Opus 5)

[spuru9]

cc: @ferenc-csaky @1996fanrui Can you help me review/merge this PR.

[1996fanrui]

thanks for the fix, merging as you have tested locally