docs(example): add README for example/json-builder (issue #275)

[Bhumika-1432006]

Part of #275 (one directory per PR; this one covers example/json-builder).

Problem

example/json-builder has no README. Issue #275 lists it among the example directories that have none.

Fix

Add example/json-builder/README.md: how to build and run step-1-basic, its output, and how each part builds its JSON (flat object, a nested object appended by hand to the builder buffer, and an array of objects). It links to docs/api/json.md.

Verification

• Built with make -C example/json-builder/step-1-basic and ran it: exit status 0, output matches the README.
• Checked by running a small program: a string value containing a quote and a newline is written as \" and \n, a NULL string value is written as null, and cwist_json_begin_array with a key followed by cwist_json_add_string with a NULL key produces "tags":["a","b"] with commas inserted by the builder.
• The header has no begin_object that takes a key, which is why the example appends the nested object as raw text; the README says so.
• ASCII check on the new file: clean.

Scope

• No source changes.
• Only example/json-builder is covered. The other directories listed in [docs/example] Add README files to the example directories #275 are left for separate PRs.