docs(hydroflow_plus): polish quickstart and drop stale pages (#1576)

I've decided to leave some more challenging TODOs as comments for now,
just so we can get the live site back into decent shape.

Also changes the "Get Started" button on the landing to point to the HF+
quickstart.

docs/docs/hydroflow_plus/dataflow-programming.mdx

@@ -0,0 +1,13 @@
+---
+sidebar_position: 1
+---
+
+# Dataflow Programming
+Hydroflow+ uses a dataflow programming model, which will be familiar if you have used APIs like Rust iterators. Instead of using RPCs or async/await to describe distributed computation, Hydroflow+ instead uses **asynchronous streams**, which represent data arriving over time. Streams can represent a series of asynchronous events (e.g. inbound network requests) or a sequence of data items.
+
+Programs in Hydroflow+ describe how to **transform** entire collections of data using operators such as `map` (transforming elements one by one), `fold` (aggregating elements into a single value), or `join` (combining elements from multiple streams on matching keys).
+
+If you are familiar with Spark, Flink or Pandas, you will find Hydroflow+ syntax familiar. However, note well that the semantics for asynchronous streams in Hydroflow+ differ significantly from bulk analytics systems like those above. In particular, Hydroflow+ uses the type system to distinguish between bounded streams (originating from finite data) and unbounded streams (originated from asynchronous input). Moreover, Hydroflow+ is designed to handle asynchronous streams of small, independent events very efficiently.
+
+<!-- TODO(shadaj): link to collections section -->
+<!-- TODO(shadaj): show example of mermaid graph -->

docs/docs/hydroflow_plus/index.mdx

@@ -3,11 +3,11 @@ sidebar_position: 0
 ---
 
 # Introduction
-Hydroflow+ is a high-level distributed streaming framework for Rust powered by the [Hydroflow runtime](../hydroflow/index.mdx). Unlike traditional architectures such as actors or RPCs, Hydroflow+ offers _choreographic_ APIs, where expressions and functions can describe computation that takes place across many locations. It also integrates with [Hydro Deploy](../deploy/index.md) to make it easy to deploy and run Hydroflow+ programs to the cloud.
+Hydroflow+ is a high-level distributed programming framework for Rust powered by the [Hydroflow runtime](../hydroflow/index.mdx). Unlike traditional architectures such as actors or RPCs, Hydroflow+ offers _choreographic_ APIs, where expressions and functions can describe computation that takes place across many locations. It also integrates with [Hydro Deploy](../deploy/index.md) to make it easy to deploy and run Hydroflow+ programs to the cloud.
 
 Hydroflow+ uses a two-stage compilation approach. HF+ programs are standard Rust programs, which first run on the developer's laptop to generate a _deployment plan_. This plan is then compiled to individual binaries for each machine in the distributed system (enabling zero-overhead abstractions), and are then deployed to the cloud using the generated plan along with specifications of cloud resources.
 
-Hydroflow+ has been used to write a variety of high-performance distributed system, including implementations of classic distributed protocols such as two-phase commit and Paxos. Work is ongoing to develop a distributed systems standard library that will offer these protocols and more as reusable components.
+Hydroflow+ has been used to write a variety of high-performance distributed systems, including implementations of classic distributed protocols such as two-phase commit and Paxos. Work is ongoing to develop a distributed systems standard library that will offer these protocols and more as reusable components.
 
 :::caution
 

docs/docs/hydroflow_plus/quickstart/_category_.json

@@ -1,6 +1,6 @@
 {
   "label": "Quickstart",
-  "position": 1,
+  "position": 2,
   "link": {
     "type": "doc",
     "id": "hydroflow_plus/quickstart/index"