🛸 Update extractParts to pull implicit parts based on heading

[fwkoch]

Parts like abstract, acknowledgements, etc. can be tagged and picked up by templates to be used separately from the rest of the content.

Currently there are two ways of defining these parts - (1) putting them under parts in the frontmatter:

---
parts:
  abstract: My abstract!
---

# Introduction

...

or (2) adding metadata to a block with inline JSON:

+++ { "part": "abstract" }

My abstract

+++

# Introduction

...

Both of these require a bit of boilerplate, but worse, if the source for your MyST build is LaTeX, these syntaxes are not available, so you cannot tag a part.

---

This PR updates the extractPart method to pull out parts based on sections. It looks for headings that match part names and, if found, extracts the subsequent paragraphs as the part. That means you can simply do something like

# Abstract

My Abstract

# Introduction

...

[rowanc1]

Works well - will need a bump in the theme to get it over there!