docs(ui-sdk): expand Server Customisations guides

[govindavashishtha]

Note

Low Risk
Documentation-only changes with no SDK or server behavior impact. Minor risk of broken internal doc links to removed TrueFoundry composition sections.

Overview
This PR restructures the UI SDK “Server Customisations” docs and adds a Bring your own server guide series, without changing runtime code.

Navigation (docs.json) adds an Overview, Connect with TrueForge, and a nested Bring your own server group (core-concepts, chat, catalog, agents, putting-it-together) ahead of Server contract. Connect with TrueFoundry stays a sibling entry.

New pages walk through choosing a server config (TrueForge / TrueFoundry / raw AgentUIServer), TrueForge auth and optional catalog, sessions/turns/events, AgentChatServer and AgentBuilderServer cookbooks, catalog attachment, and layering chat + builder + catalog into one server.

Updated guides: Connect with TrueFoundry is trimmed to the minimal type: "truefoundry" example (long createTrueFoundryAgentUIServer / createTrueFoundryServer sections removed). Bring your own server drops the TrueFoundry-only note, adds attachment override and a “What’s next” card grid, and moves type-widening detail to Server contract.

Cross-links across quickstart, architecture, troubleshooting, reference, and slot-overrides now point at ui-sdk/setup-custom-servers/* and ui-sdk/setup-custom-ui/* instead of older guides/ and concepts/ paths.

Note: putting-it-together and custom-server still link to #truefoundry-chat-your-own-builder on Connect with TrueFoundry, but that anchor was removed in this PR—those links may 404 until restored or updated.

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