Document publishing and link the store walkthrough

[diegoparrilla]

CLAUDE.md covers building an app thoroughly and then stops at dist/. It never says that desc/app.json is the source of the generated descriptor, that three of its fields are placeholders build.sh fills in, or how a built app reaches the catalogue Booster downloads from. An assistant working in this repo has no way to know any of it.

Adds a short Publishing section immediately before the editing guardrails, next to the existing App identity material it follows from. It covers:

• what the three files in dist/ are, and that rp.uf2.md5sum is a byproduct rather than something you publish
• that the descriptor is generated from desc/app.json, and that <APP_UUID>, <APP_VERSION> and <BINARY_MD5_HASH> must stay as placeholders because build.sh substitutes them every run
• that the binary URL should carry those placeholders too, so it tracks the version across releases
• that getting listed is one pull request against the store, which keeps a pointer rather than a copy, so later releases need no further pull request

It links the step-by-step walkthrough in the store's build guide rather than restating it, so there is one copy of that process to keep correct.

Checked against this repository's build.sh before writing: the dist/ filenames, the three substituted placeholders, and the abort when desc/app.json is missing all match what the section claims.