feat(v3): add `pageSize` plugin option

[angeloashmore]

Package

• [x] gatsby-source-prismic
• [ ] gatsby-plugin-prismic-previews

Types of changes

• [ ] Chore (a non-breaking change which is related to package maintenance)
• [ ] Bug fix (a non-breaking change which fixes an issue)
• [x] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR backports the pageSize plugin option from v4+ to v3.

The pageSize plugin option is described as such in the gatsby-source-prismic v5 documentation:

Set the maximum page size used when fetching documents from the Prismic API. If you are reaching API limits due to large documents, set this to a number less than the maximum (100). By default, the maximum page size of 100 is used.

Setting pageSize to a value less than the maximum (100) is necessary in cases where the query response exceeds Prismic's Rest API maximum payload size.

Checklist:

• [x] My change requires an update to the official documentation.
• [x] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [x] All new and existing tests are passing.