[webportal doc] webportal doc broken link

[YanjieGao]

need fix broken link and direct to specific version doc

Potential proposal:

1. replace the doc here with the accordingly MD files of each release
2. Or just embed the GIT document into this page to avoid diverge
3. How about click this Getting Started button is directly open a new page of specific version of pai job md page (such as, https://github.com/Microsoft/pai/blob/v0.8.1/docs/job_tutorial.md), and not render it in pai webportal.

[Gerhut]

The root cause is that GitHub auto generated anchor is difference from marked generates. So we should clarify the anchor generating logic of the whole doc.

[YanjieGao]

currently this doc will use some relative path for refer's pai's other doc.
such as:

and table of content also not work: