Point README and CONTRIBUTING at the in-repo addon manual

[eduralph]

User impact: README and CONTRIBUTING now lead addon authors to the in-repo Addon Development manual, so the deep technical reference and the contributor workflow stop drifting apart — while every workflow section contributors wrote (fork setup, dev branches, the addon checklist, the PR walkthrough) stays in place, in their own words.

Depends on: the PR that adds docs/addon-development/ (the manual import). This PR only rewires the two entry documents onto it; reviewing it with that one merged (or checked out) makes the links live.

Summary

• README.md — the "develop your own addon" pointer now leads to docs/addon-development/README.md first, with CONTRIBUTING.md named for the workflow and the wiki page kept as an alternative rendering. The dead Travis badge is dropped. Nothing else changes.
• CONTRIBUTING.md — becomes the contributor workflow guide it already mostly was. Sections whose technical depth the manual now covers (addon kinds, .gpr.py registration + GENERAL plugins, prerequisites, configuration, localization, distribution contents, report categories, the wiki listing/documentation templates) are reduced to a short retained summary plus a link into the manual — each under its original heading, so existing deep links and the ToC keep resolving. The unique workflow content is untouched: GitHub/fork setup, the three-repo dev environment, development branches, Review the Addon Checklist, the whole Create a Pull Request walkthrough, bug-tracker support, maintenance guidance, and "Addons External to Github".

What to look at

• The per-section judgment calls: is the retained summary in each slimmed section the right 2–4 lines to keep inline? The diff is heading-by-heading, so each disposition reads independently.
• Drive-by repairs of pre-existing broken links, all verifiable in the diff: seven ToC/overview anchors that never matched their headings (e.g. #setup-a-github-account vs "Set Up a Github Account"), two (#https://...) hrefs, the garbled Addon list legend]]] link, and the Localization snippet that had lost its _ in _ = glocale.get_addon_translator(...).

Verification

• Every internal #anchor in CONTRIBUTING.md now resolves to a heading (checked mechanically with a GitHub-slug generator; zero missing, previously seven).
• Every docs/addon-development/... link and #anchor in both files resolves against the manual tree from the dependency PR (checked mechanically).
• No heading was added, removed, or renamed in CONTRIBUTING.md — the ToC structure is byte-identical.