Documentation for extension maintainers

[asgrim]

Design documentation: https://github.com/ThePHPF/pie-design/blob/main/README.md

Produce documentation for extension maintainers on how to make their extension available with PIE.

[asgrim]

Things of note that have come up, to ensure are covered in the docs:

• how the configure-options work, especially around needs-value flag (ref: PHPC-2420: Initial composer.json for pie support mongodb/mongo-php-driver#1624 (comment))
• there is no default values for configure options; they are either provided by the user, or not
• it is worth noting that a php extension cannot share the same Composer package name as a regular php package (ref: PHPC-2420: Initial composer.json for pie support mongodb/mongo-php-driver#1624 (comment))