Add UI section to the docs

[blarsen]

Summary

• Adds a new UI chapter to the book, with an overview page and one page per screen, in the order of the navigation bar: Home, Torrents, Events, Search, Goodreads lists, Errors, Selected Torrents, Replaced Torrents, Duplicate Torrents and Config.
• Written for end users: each page describes what is on the screen and what the buttons do.
• Pages live in docs/src/ui/ and are listed in docs/src/SUMMARY.md.

Test plan

• mdbook build succeeds
• Served locally with mdbook serve and checked the new pages load
• Descriptions read against the running UI

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