docs: fix deno create page to require jsr:/npm: prefix

[lunadogbot]

Summary

The deno create page claimed unprefixed package names default to JSR, but on Deno 2.7.14 the CLI errors out with Missing 'jsr:' or 'npm:' prefix. for any unprefixed argument (e.g. deno create @fresh/init, the page's own example, fails). A prefix (jsr: / npm:) or flag (--jsr / --npm) is required.

This brings the docs in line with current CLI behavior:

• Rewrote the intro paragraph to say a prefix or flag is required, and quote the actual error.
• Updated the JSR examples (deno create @fresh/init → deno create jsr:@fresh/init, including the -- --force variant and the inline @my-scope/my-template reference in the "How it works" section).
• Dropped the (default) annotation from the --jsr flag — there is no default.

If the CLI later changes to actually default to JSR, the docs can be reverted to match — that's a runtime change tracked elsewhere.

Closes #3061
Closes bartlomieju/orchid-inbox#49

cc @bartlomieju

Test plan

• deno create @fresh/init against denoland/deno:latest (2.7.14) errors with Missing 'jsr:' or 'npm:' prefix. — matches the updated docs.
• deno create jsr:@fresh/init and deno create --jsr @fresh/init proceed past the prefix check.
• Render the page locally and confirm the examples are copy-pasteable.