Prevent duplicate chapter title and first in-page nav item

[RobjS]

For page-break based long reads, we have to assume that the first heading block after a page break is the title of that page/chapter.

However, we assemble the in-page nav by getting all the h2 heading blocks within that chapter.

This meant that if the first heading after a page break was an h2, that heading would be used as both the title of the chapter in the side nav, and as the first in-page nav item of that chapter.

This change checks if the title of the first in-page nav item matches the chapter title, and if so, it removes that in-page nav item.

Resolves: #44

How to test

1. Checkout the main branch
2. Create a page-break based long read, where the first heading block in the second page is an h2
3. Observe that that h2 is rendered in the side nav as both the title of that chapter and the first in-page nav item of that chapter
4. Checkout this branch
5. The heading should now only be used as the chapter title, and the in-page nav items should start with the second heading in the page

Checklist

• Changelog updated
• If new release: major version tag to be bumped after release (see docs)

[matthewpassmore]

Works as expected 👍

Ignore my flagged issues in the previous PR. I didn't realise that an env var needed setting 🤦

[RobjS]

Works as expected 👍

Ignore my flagged issues in the previous PR. I didn't realise that an env var needed setting 🤦

I should have made that clearer in the testing instructions, sorry!