Add anchor links to docs

[jpoehnelt]

Is your feature request related to a problem? Please describe.

I want to be able to link to a specific section of the documenation.

Describe the solution you'd like

All h1, h2,... tags have an id attribute.

Describe alternatives you've considered

Linking to just the page.

Additional context

n/a

[mrdoob]

Can you share one example of what you want a link to?

[jpoehnelt]

Compatibility section of https://threejs.org/docs/#manual/en/introduction/Installation.

[mrdoob]

Ah, right. That's the "old" manual...

[tasamai]

Might find a weekend to get this done

[mukul-kr]

so do I have to make something which looks like this ?

and if it is what is required then do I have to do this in side bar or in the page ?