Update overview docs

[drewhoskins-temporal]

What does this PR do?

Revamp message passing overview docs for the Public Preview launch of Workflow Update with the following goals.

• Teach users about workflow concurrency and how to make safe hanadlers.
• Correct misstatements about Update from the pre-release.
• Help users choose whether to use signals or updates.

Test plan

http://localhost:3000/encyclopedia/workflow-message-passing Click all the links.

Notes to reviewers

• This was reviewed by Engineers and @axfelix in https://docs.google.com/document/d/1XsCgi7ibqxanDEfh2nS19b_4KBdZajiqtpmyy_cpErc/edit
• A few of the links are waiting for the language-specific docs to create the #subsections, to be checked in in separate PRs before Update launches in Public preview. They currently nav to the top of the page, which I hope is acceptable until we make those commits.
• According to @napcs, the image needs to be redrawn in the house style.

[drewhoskins-temporal]

I believe I've addressed or considered all the feedback, and I've uploaded a new image built by the docs team + @dandavison Logistics note: I'm going on vacation tomorrow, and we would like to get this in so that the engineers can build and test the per-language docs that link back and forth with this with confidence. We're going to do another update to these docs in the coming days/weeks as we ship public preview (deadline Aug 6). I also imagine engineers will want to change bits of this when they build the per-language docs. I welcome as much more feedback as people have, but hope that it can be addressed in the next PR unless it is a shipstopper.

[axfelix]

PS -- in the future, we need to get @drewhoskins-temporal a Vercel seat, and I'd strongly encourage making branches on this repo rather than on forks -- both of these will make actually generating previews much easier.

[axfelix]

And FYI, the build is failing right now (due to a bad link), which is exactly why we like to do work on this repo rather than on forks and need to make sure everyone who's going to work on docs has a Vercel seat. @drewhoskins-temporal , as it stands I would have to open a PR on your fork to fix this before merging, which is not ideal — do you want me to do that, or can you take a look at the failure and sort it out?

[dandavison]

I've updated the diagram to move the "Signals, Updates, Queries" arrow to the left rather than the right (It's better practice to have causality flowing left-to-right).

Here's a screenshot; the original is in lucidchart.