Add plugin architecture doc

[Dinamiko]

This PR adds a new document explaining the plugin architecture.

[Narek13]

Thanks for working on this. I left couple of suggestions, mostly about removing any references to code line numbers, as in case of changes, we would need to update the docs as well

[Dinamiko]

@Narek13 thanks for the feedback, all line numbers are removed also I updated the modularity library name.

[AlexP11223]

Line numbers can be tied to a commit when they are really needed (Copy Permalink). But it makes more sense for huge files.

Syde Modularity framework

Fun fact: it's still Inpsyde in its readme 🐢 https://github.com/inpsyde/modularity

[AlexP11223]

This sounds a bit confusing if talking about class_exists( '\WooCommerce\PayPalCommerce\PluginModule' ). Maybe like this would be more clear:

Suggested change

	- Loads the Composer autoloader
	- Checks for class existence to prevent conflicts
	- Loads the Composer autoloader if needed (e.g. may be already loaded in some tests)

[AlexP11223]

Maybe should add that it starts the bootstrap process, in plugins_loaded hook.

[AlexP11223]

Modules already have the container (and usually pass it where needed). PPCP:: is more about some third-party interop, such as api/order-functions.php or some tests.

[AlexP11223]

Suggested change

	Most modules implement the Inpsyde Modularity interfaces (`modules/ppcp-api-client/src/ApiModule.php`):
	Most modules implement the Inpsyde Modularity interfaces. For example in `modules/ppcp-api-client/src/ApiModule.php`:

[AlexP11223]

I think webhooks fit more into core, iirc some things even will not work without them.

[AlexP11223]

Suggested change

	// In modules with container access
	$service = $container->get( 'service.id' );
	// In WordPress hooks after plugin initialization
	$service = PPCP::container()->get( 'service.id' );
	// In our modules/services/extensions (also often passed to hook handlers via `use`)
	$service = $container->get( 'service.id' );
	// In third-party plugins etc. (if not adding a custom module via the `woocommerce_paypal_payments_modules` filter)
	$service = PPCP::container()->get( 'service.id' );

[Dinamiko]

@AlexP11223 thanks for the feedback, content is updated.