docs: frame mado as a Markdown reader, group the features, add a Japanese README

[hidekingerz]

Summary

mado has grown past "a TUI markdown viewer", and the README now says so.

• Positioning: the opening now describes mado as a terminal reader for the Markdown in a directory (a project's docs, a notes folder), written in Go on Bubble Tea and Glamour, with the file tree, tabs, search and diagrams as parts of reading rather than a feature pile.
• Features grouped into four themes instead of fourteen flat bullets: Reading, Finding your way, Alongside other tools, Making it yours. The "content is shown, never obeyed" safety point and line numbers are surfaced there; they were only in the body before.
• Usage subsections reordered to follow the same themes. Their content is unchanged: every removed line was re-added verbatim except the two opening lines.
• README.ja.md: a full Japanese translation with the same sections in the same order. Both files link to each other at the top.

Test plan

• Diffed removed vs. added lines of README.md: only the two intro lines changed in content
• README.ja.md matches README.md in heading count (19), fenced code block count and key-table rows; key names and defaults checked against config.example.toml and internal/ui/keys.go
• Rendered both files in mado

🤖 Generated with Claude Code

https://claude.ai/code/session_01KUYF9hXeZFML1s8r3i2RVX