Rewrite the xchat cryptography primer

[alazzaretti]

Rewrites /xchat/cryptography-primer for clarity and accuracy.

• Restructures the page around the lifecycle of the keys (account creation, conversation creation, messaging, signing), with a worked example of creating a group.
• Uses plain language in the body; the exact schemes (ECIES, ECDH, ECDSA, P-256) live in the glossary.
• Expands secure key backup: names the open-source Juicebox protocol, describes the 2-of-3 realm topology with two HSM-backed realms and the 20-attempt guess limit with share deletion, and notes that every recovery path goes through an HSM.
• States security properties together with the assumptions they rest on, and states the limitations (metadata, no forward secrecy, deliberate rather than automatic post-compromise recovery) plainly, with reasons.

Tested locally with mint dev: page renders, and all internal links and anchors were verified.

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[mintlify]

Docs PR opened: #276

Rewrote the X Chat cryptography primer's title and description to be more specific, descriptive, and SEO-friendly.

[mintlify]

Docs PR opened: #277

Updated the cryptography primer's title and sidebar title from title case to sentence case for consistency with sibling pages.

Note: failed to automatically merge

[mintlify]

Docs PR opened: #278

Translated the rewritten X Chat cryptography primer into Japanese, Spanish, Portuguese, and Korean.