docs: restructure all four READMEs and document the sync rule (#931)

* docs: restructure README into grouped sections

Reorganize the README around six top-level sections (Features,
Installation, Usage, Configuration, Docker, Q&A, Development) so
related content is no longer scattered:

- add a table of contents
- move the long Motivation section to the end and lead with a
  one-line description
- collect all config topics (CLI overrides, startup banner,
  entrypoint, env files, platform overrides, watch rules) under
  Configuration
- collect all Docker topics (official image, shell function,
  Compose, custom Dockerfile) under Docker
- put each badge on its own line
- fix the missing line continuation in the docker run example,
  which made `-c <CONF>` parse as a separate command
- normalize Q&A headings, trailing whitespace and long lines

* docs: follow gin's README layout conventions

Align the structure with gin's README:

- `## Contents` nested TOC (including the title and Contents
  itself), one entry per heading
- Installation first, then a Quick start section with the minimal
  run/init flow
- Features as a checklist
- expand the collapsed <details> Docker examples inline; gin keeps
  everything expanded
- drop bare-link headings in favour of plain heading text with the
  link in the body

All 39 TOC anchors verified against the headings.

* docs: document basic proxy usage

Add a Configuration subsection covering the browser live-reload
proxy: minimal [proxy] config, the fact that you open proxy_port
rather than app_port, the </body> injection requirement, and
app_start_timeout for slow-booting apps.

The Q&A entry on static-file reloads now points at it instead of
repeating a partial config snippet.

* docs: sync translated READMEs with the English structure

Rebuild README-zh_cn.md, README-zh_tw.md and README-ja.md on the
same gin-style skeleton as README.md: badges one per line, a
nested table of contents, Installation before Quick start, and
grouped Configuration / Docker / Q&A sections. All three now have
the identical heading shape as the English file.

Translate the sections the translations were missing entirely:
startup banner, entrypoint, environment files, platform-specific
build overrides, watch rules, and the new proxy section. Also add
the repeated list-argument note and the .env feature bullet.

Fixes carried over while rewriting:

- zh_cn: goblin.run install and all issue links pointed at the old
  cosmtrek/air repo; Homebrew was missing entirely; the language
  switcher had no Japanese entry; the Star History image used the
  retired svg endpoint
- zh_tw: a duplicate '透過 go install' section, a missing closing
  backtick in the WSL answer, and two Q&A entries that were never
  translated
- ja: the pilu link had a typo (https:///github.com)
- all: the docker run example was missing the line continuation
  before -c <CONF>

TOC anchors generated and verified against the headings in each
file (40 headings, 42 links, no broken anchors per file).

* docs: require translated READMEs to be updated in the same change

README.md is the source of truth and the three translations are
routinely left behind — before this branch they were missing the
startup banner, entrypoint, env files, platform override and watch
rule sections entirely.

Document the rule in AGENTS.md: any section added, removed, renamed
or reordered in README.md must be mirrored in README-zh_cn.md,
README-zh_tw.md and README-ja.md in the same change, the four files
share one heading skeleton, and the table of contents must stay in
sync. Includes a shell one-liner that prints the heading shape of
each file so parity can be checked before committing.