docs: #10221 Create 404 page (#10296)

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## Description

Closes: #10221

Added 404.md file with the right settings.  Not adding any graphic design/fancy element until it can be verified to work properly on the official site, at which point graphical flourishes will be trivial.

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [NA ] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [NA] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [NA] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [NA] added a changelog entry to `CHANGELOG.md`
- [NA] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [NA] reviewed "Files changed" and left comments if necessary
- [NA] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

docs/.vuepress/config.js

@@ -58,7 +58,7 @@ module.exports = {
     topbar: {
       banner: true
     },
-    sidebar: { 
+    sidebar: {
       auto: true,
       nav: [
         {

docs/404.md

@@ -0,0 +1,49 @@
+<!--
+permalink: /404.html
+layout: homepage
+title: 404 Lost in Space!
+description: You've been lost in space
+sections:
+  - title: Introduction
+    desc: High-level overview of the Cosmos SDK.
+    url: /intro/overview.html
+    icon: introduction
+  - title: Basics
+    desc: Anatomy of a blockchain, transaction lifecycle, accounts and more.
+    icon: basics
+    url: /basics/app-anatomy.html
+  - title: Core Concepts
+    desc: Read about the core concepts like baseapp, the store, or the server.
+    icon: core
+    url: /core/baseapp.html
+  - title: Building Modules
+    desc: Discover how to build modules for the Cosmos SDK.
+    icon: modules
+    url: /building-modules/intro.html
+  - title: Running a Node
+    desc: Running and interacting with nodes using the CLI and API.
+    icon: interfaces
+    url: /run-node/
+  - title: Modules
+    desc: Explore existing modules to build your application with.
+    icon: specifications
+    url: /modules/
+stack:
+  - title: Cosmos Hub
+    desc: The first of thousands of interconnected blockchains on the Cosmos Network.
+    color: "#BA3FD9"
+    label: hub
+    url: http://hub.cosmos.network
+  - title: Tendermint Core
+    desc: The leading BFT engine for building blockchains, powering Cosmos SDK.
+    color: "#00BB00"
+    label: core
+    url: http://docs.tendermint.com
+footer:
+  newsletter: false
+aside: false
+-->
+
+# 404 - Lost in space, this is just an empty void...
+
+