docs: multi-publication documentation site (#18912)

This PR sets the groundwork for deploying multiple docs publications in
a single site.

# Overview

This PR sets up the
[mkdocs-monorepo-plugin](https://github.com/backstage/mkdocs-monorepo-plugin)
to configure a single mkdocs project that includes smaller mkdocs
projects in their own subdirectories inside of `docs/`.

Publications included in this PR so far:
- Flex manual
- Stacker manual
- HEPA/UV manual

We'll add more publications in future PRs to keep reviews manageable.

## Test Plan and Hands on Testing

There is no CI/sandbox tooling associated with this new setup yet. Test
the docs locally:
- `pip install mkdocs-monorepo-plugin`
- `mkdocs serve` in the top-level `docs/` directory

## Changelog

Lots.

- monorepo docs configuration
- some edits to Stacker manual
- brand new conversion of HEPA/UV manual to mkdocs
- placeholders for PAPI and PD docs

## Review requests

Does this seem functional? If so, let's get it merged and keep building
on it.

## Risk assessment

low-mid. there could be some merge conflicts with open PRs to modify the
Flex manual. if so, i can handle those conflicts post-merge.

Author: ecormany

docs/flex-manual/docs/additional-documentation.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Additional Documentation"
+---
+
 # Additional Documentation
 
 Opentrons maintains additional online documentation for our hardware and software products. You may find these resources valuable as you use Opentrons Flex.

docs/flex-manual/docs/glossary.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Glossary"
+---
+
 # Glossary
 
 This appendix defines terms related to Opentrons Flex. It omits industry-standard terms like "labware" unless the term has a special meaning in relation to Flex. For example, the definition for *pipette* describes the narrower meaning that the term has when using Flex, as opposed to any pipette you might find elsewhere in a lab.

docs/flex-manual/docs/installation-relocation.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Installation and Relocation"
+---
+
 # Installation and Relocation
 
 This chapter describes how to prepare your lab for Opentrons Flex, how to set up the robot, and how to move it if necessary. Before taking delivery of your Flex, make sure that your lab or facility meets all the criteria in the Safety and Operating Requirements section. When it's time to get your Flex up and running, follow the detailed instructions in the Unboxing, First Run, and Instrument Installation and Calibration sections, or use the Opentrons [On-Site Installation service](https://shop.opentrons.com/onsite-support-set-up/). And if you ever need to move your Flex to a new location, near or far, follow the steps in the Relocation section.

docs/flex-manual/docs/introduction.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Introduction"
+---
+
 # Introduction
 
 This chapter introduces you to the Opentrons Flex ecosystem, including the overall system design and available workstation configurations. It also includes important compliance and safety information, which you should review before setting up your Opentrons Flex robot. For more details on the features of Opentrons Flex, see the [System Description chapter](system-description.md).

docs/flex-manual/docs/labware.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Labware"
+---
+
 # Labware 
 
 This chapter covers items in the [Opentrons Labware Library](https://labware.opentrons.com/) you can use with Opentrons Flex and the Opentrons Flex Gripper. It also covers custom labware and, for our power users, links labware components to their corresponding JSON file definitions. 

docs/flex-manual/docs/maintenance-service.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Maintenance and Service"
+---
+
 # Maintenance and Service
 
 This chapter covers how to perform routine maintenance on your Opentrons Flex, and what to do if you require service for a problem. You should be able to perform cleaning tasks yourself, whereas service and repairs should be handled by Opentrons Support.

docs/flex-manual/docs/manual.css

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Modules"
+---
+
 # Modules
 
 Opentrons Flex integrates with several Opentrons hardware modules that add features and capabilities to the robot. Modules can occupy deck slots or are external, frame-mounted components. Flex communicates with and controls most modules via a USB connection.

docs/flex-manual/docs/modules.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Open-Source Software"
+---
+
 # Open-Source Software
 
 Opentrons believes that open-source software and hardware make science

docs/flex-manual/docs/open-source-software.md

@@ -1,3 +1,7 @@
+---
+title: "Opentrons Flex: Preface"
+---
+
 # Preface
 
 Welcome to the instruction manual for the Opentrons Flex liquid handling robot. This manual guides you through just about everything you need to know to set up and use Flex, focusing on topics that are most relevant to everyday users of Flex in a lab environment.