tools: improve docopen target in Makefile

Checklist

• commit message follows commit guidelines

Affected core subsystem(s)

tools Makefile

Description of change

1. As it is, it just tries to build only the all.html file. If none of the other files are built already, generated page will not be good. To fix this, we process the assets and generate HTML files first.

2. After the HTML is generated, google-chrome is used to open the generated file in browser. This is not very portable as it might not be installed or installations might have used a different name. So, we use Python's webbrowser module to open the file.

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cc @nodejs/documentation @nodejs/build