Merge tutorial and porting guide into user guide

[qinsoon]

This PR merges the current tutorial and the current porting guide into one mdbook: user guide. This PR fixes related links and CI scripts, but does not change the wording or the structure for the docs. Further PRs on improving the docs are welcome.

[caizixian]

Should we also target this for the 0.19 release?

[qinsoon]

Should we also target this for the 0.19 release?

I don't know if it matters, as most people don't read the docs from the repo. Any reason we should or should not include this in the current 0.19 release? If there is no particular reason, we can be relaxed on when to merge this.

[caizixian]

Should we also target this for the 0.19 release?

I don't know if it matters, as most people don't read the docs from the repo. Any reason we should or should not include this in the current 0.19 release? If there is no particular reason, we can be relaxed on when to merge this.

It's not super urgent, but I think because when we make a new release, the README of the repo is persisted on crates.io and docs.rs, so it would be nice if we can fix up the links.

[caizixian]

I think this mostly addresses our documentation discoverability problem. Once merged, we can update the mmtk website, and gradually add other documentation to the user guide, such as the tracing documentation.