📝 Refresh XDR Formatting Information

[JFWooten4]

This adds some needed SEO descriptions, cleans up the sidebar, and keeps the links directed.

[Copilot]

Pull request overview

Refreshes the “Data Format” docs section to improve SEO metadata and tighten up documentation linking/structure around XDR and XDR-JSON.

Changes:

• Added page-level SEO description frontmatter to the XDR and XDR-JSON docs.
• Added a _category_.json to define the “Data Format” sidebar category and link it to the section README.
• Updated/normalized a few links (RFC 5531 section link + inline stellar-cli link) and tweaked copy in the RPC data-format page.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
docs/learn/fundamentals/data-format/xdr.mdx	Adds SEO description; converts record-marking references to direct links; inlines stellar-cli link.
docs/learn/fundamentals/data-format/xdr-json.mdx	Adds SEO description frontmatter.
docs/learn/fundamentals/data-format/category.json	Introduces sidebar category metadata and links the category to the section README doc.
docs/data/apis/rpc/api-reference/structure/data-format.mdx	Minor copy update in the XDR decoding sentence.

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[ElliotFriend]

same not as on #2446 about the category json file. it'll lead to a conflict if we include it in this PR. otherwise, this is a pretty easy merge, once the sidebar position is fixed.

[ElliotFriend]

this will move the Data Format category up to the very top of the Learn -> Fundamentals sidebar. i don't think that's what we want?