docs: restructure the README and move the details to docs/

[lesnik512]

The README had grown to 2,600 words, and most of it was accurate detail in the wrong place: a 13-line paragraph on how collection works before the reader learned what the tool is for, failure semantics inside the Quickstart, and about 1,100 words narrating the page that the demo now shows.

README

It now opens with what the tool shows and who it is for, then the demo link and a screenshot of the demo with SHOP-38 chosen. After that, the order follows what a new user does:

1. Quickstart, now with serving the page.
2. Publish with GitLab Pages (moved up from near the end).
3. Choosing a release: a short how-to for the page.
4. Jira.
5. Configuration (unchanged).
6. Failures and exit codes: new, and it documents exit codes 2, 3 and 4, which were missing.
7. Cache.
8. Agent skill.
9. Links to the detail pages.

All links stay absolute, so the PyPI page keeps working. The screenshot loads from raw.githubusercontent.com/.../main/docs/images/demo.png, so it shows only after merge.

Moved to docs/

• docs/how-it-works.md: the range, rows, Jira links and link states, --jira scoping with a table of release states, and the next-tag rule.
• docs/page.md: the full page behaviour, split into sections, with a table of the issue reach labels.
• docs/report.md: the report.json fields and the sample row.

The text was rewritten into shorter sentences and lists, with every fact kept, and run through humanizer. Local links pass the same offline lychee check CI runs.