[Doc] Clarify README install and quick start guidance

[minggangw]

• separate npm installation from repo checkout quick start
• make ROS 2 environment sourcing explicit in prerequisites and examples
• add direct GitHub install instructions
• clarify that GitHub installs usually build from source
• limit prebuilt binary claims to the published npm package
• improve docs and message-generation wording
• align npmjs README with the updated package-consumer guidance

Fix: #1452

[Copilot]

Pull request overview

Clarifies install vs. quick-start guidance in the project README and the npmjs-specific README, aiming to reduce confusion around environment setup, install sources (npm vs GitHub), and message generation.

Changes:

• Adds explicit ROS 2 environment “source” guidance and separates installation paths (npm dependency vs repo quick start).
• Reworks GitHub install instructions into concrete npm install RobotWebTools/rclnodejs#... commands.
• Expands documentation links (adds tutorials link in the npmjs README) and refines message-generation wording.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
scripts/npmjs-readme.md	Clarifies prerequisites (sourcing ROS 2), GitHub install command, docs links, and message-generation guidance for the npm page.
README.md	Reorganizes installation/quick-start instructions, updates anchors/headings, and refines docs guidance.

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[Copilot]

The statement that prebuilt binaries apply when installing from GitHub appears incorrect. scripts/install.js only checks for a local prebuilds/<platform>-<arch>/... file and does not download prebuilds; since the prebuilds/ directory is not present in the repo, npm install RobotWebTools/rclnodejs#... will fall back to building from source. Consider limiting this claim to npm installs (published tarballs) or documenting that GitHub installs build from source unless the branch includes prebuilds/ artifacts.

Suggested change

	rclnodejs ships with prebuilt native binaries for common Linux configurations since `v1.5.2`, eliminating the need for compilation during installation. This applies to supported Linux environments when installing from npm or GitHub.
	rclnodejs ships with prebuilt native binaries for common Linux configurations since `v1.5.2`, eliminating the need for compilation during installation when installing from npm. These prebuilds are bundled in the published npm package tarballs.
	GitHub installs (for example, `npm install RobotWebTools/rclnodejs#<branch>`) build from source by default unless that branch or tag includes `prebuilds/` artifacts for your platform.

[Copilot]

This sentence implies GitHub installs get prebuilt native binaries, but the current install flow (scripts/install.js) only uses prebuilds if a prebuilds/ directory exists locally and does not fetch them. Since prebuilds/ is not present in the repo, npm install RobotWebTools/rclnodejs#... will typically build from source. Please adjust the wording to avoid promising prebuilt binaries for GitHub installs (or explain the required conditions).

Suggested change

rclnodejs ships with prebuilt native binaries for common Linux configurations since `v1.5.2`, eliminating the need for compilation during installation. This applies to supported Linux environments when installing from npm or GitHub.

rclnodejs ships with prebuilt native binaries for common Linux configurations since `v1.5.2`, eliminating the need for compilation during installation when a matching prebuilt is available. In typical setups, this applies to supported Linux environments when installing from npm. When installing directly from GitHub (for example using `npm install RobotWebTools/rclnodejs#<branch>`), the package will usually be built from source unless your install process provides compatible prebuilt binaries locally.

[coveralls]

coverage: 85.995% (+0.02%) from 85.976%
when pulling 3cd4fa7 on minggangw:fix-1452
into 6e46c4e on RobotWebTools:develop.