SDK-1486: Python update docs

[dandavison]

[rendered]

This is a new version of the message-passing documentation for Python. It follows on from the new top-level section on https://docs.temporal.io/encyclopedia/workflow-message-passing that @drewhoskins-temporal recently wrote. The significant changes here are:

• There is now a code sample at the top of the page that will immediately give many visitors to the page what they're looking for: it is minimal, but it shows how to define Query, Signal, and Update handlers, and an Update validator, and has one-line comments introducing the concepts.

• Following the style taken in https://docs.temporal.io/encyclopedia/workflow-message-passing, the document recognizes that many aspects of Signal and Update handling can be covered together. This avoids a lot of repetition.

• In particular, a new section named Message handler patterns gives Python-specific discussion and examples of the content outlined in encyclopedia/workflow-message-passing.

• Similarly, there was no need to discuss dynamic handlers separately for Signal, Query, and Update since the signatures are the same.

The structure of the document is (I'll keep this up-to-date it it changes)

## Writing message handlers
  ### Query handlers {#handle-query}
  ### Signal handlers {#handle-signal}
  ### Update handlers and validators {#handle-update}

## Sending messages
  ### Sending a Query {#send-query}
  ### Sending a Signal
    #### Sending a Signal from a Client {#send-signal-from-client}
    #### Sending a Signal from a Workflow {#send-signal-from-workflow}
    #### Signal-With-Start {#signal-with-start}
  ### Sending an Update {#send-update}
    #### 1. Use [`execute_update`](https://python.temporal.io/temporalio.client.WorkflowHandle.html#execute_update) to wait for the update to complete
    #### 2. Use [`start_update`](https://python.temporal.io/temporalio.client.WorkflowHandle.html#start_update) to receive a handle as soon as the update is accepted or rejected
    #### Non-type safe APIs

## Message handler patterns {#message-handler-patterns}
  ### Async handlers {#async-handlers}
    #### Waiting
    #### Use `asyncio.Lock` to prevent concurrent handler execution {#control-handler-concurrency}
    #### Finishing handlers before the Workflow completes {#wait-for-message-handlers}

## Troubleshooting
  ### Problems when sending a Signal
  ### Problems when sending an Update
  ### Problems when sending a Query
## Dynamic Handlers {#dynamic-handler}
  ### Set a Dynamic Signal, Query, or Update handler {#set-a-dynamic-signal}
  ### Set a Dynamic Workflow {#set-a-dynamic-workflow}
  ### Set a Dynamic Activity {#set-a-dynamic-activity}

TODO:

• get_update_info()
• add "workflow execution already completed" code

[drewhoskins-temporal]

Looks good, mostly minor comments. Main question is just to refine our posture on how much explanatory text goes here vs in the overview docs, and how we link to the overview docs, which I've flagged in a scattershot fashion in the inline comments.
It's convenient to have things here for people who like to dive right in. But I'm worried about a middle ground where we say some important things but not others.
I've had two experiences with temporal's language specific docs where they didn't answer my question, and I wasn't aware that the answer was in the overview docs. (that is, wasn't aware that there were overview docs on those topics and at this time I had not internalized the overall structure of the docs.)
So, liberal upward linking is probably the main thing I would recommend, and then use judgment about whether to repeat things here (and maybe overall I would recommend doing it less than you are and rely more on the links. This would ensure people are getting the best version of our intro on these topics, which we can maintain carefully.).

[cretz]

Some nits here and there, but overall LGTM

[dandavison]

Thanks for the review @drewhoskins-temporal! I believe I've responded to all your comments. I've added several more uplinks.

Turning to @cretz's review now which also looks very helpful.

[dandavison]

Is this ready to transition from SDK-1486 to EDU-671?

Yes, over to docs team!

[dandavison]

Erica's changes are merged into this one; this PR is now just waiting for an approval from docs team.

[axfelix]

Stamped! Thanks for working with us so much on this one. Really happy with how it turned out.

[dandavison]

Thanks @axfelix and @fairlydurable.

I've added two final commits for @fairlydurable to review:

1. changing a code snippet only to address an uresolved review comment above: 2820990

2. Adding a line documenting workflow.current_update_info which should be included but was not: 6446f01

I'll obviously let you know if I think of any more omissions, but other than that over to you for merge.