doc: correct module loading descriptions

[joyeecheung]

The existing description is outdated, and exposes too many details that are subject to change.

• There is no point conceptualizing "two module loaders", in reality the boundary is blurred since the two invoke each other to support require(esm) and import(cjs). The distinction lies not in what kind of module is being requested/which loader is used, but only in how the the module request is initiated (via require() or import()). The inner working of the loaders are subject to change and not suitable to be documented.
• It should not mention monkey patching in the documentation, as publicly supported universal hooks are already provided, and so there's no need to single out any of them in terms of loader hooks support either.
• Remove the description about whether they are asynchronous or synchronous, which is also implementation detail subject to change.
• Add missing descriptions about how .ts, .mts and .cts are treated, and .node is also supported in import now.
• There is no need to specially mention .node treatment in cli.md, link to the explanations about loading from import in packages.md instead.