Initial version of block template lifecycle doc

docs/product-editor-development/block-template-lifecycle.md

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+# Block template lifecycle
+
+A block template is a tree structure of blocks that define the product editor's form structure.
+
+A template can be modified by using the block template API to add new blocks (groups, sections, and fields) as well as remove existing ones.
+
+A template is implemented in PHP and sent to the client. The client then renders the template using React.
+
+The lifecycle of a template is as follows:
+
+- [Creation](#creation)
+- [Block addition and removal](#block-addition-and-removal)
+    - [Actions](#actions)
+- [Registration](#registration)
+- [Sent to client](#sent-to-client)
+- [Rendered on client](#rendered-on-client)
+
+## Creation
+
+A template instance is created by instantiating a class that implements the `Automattic\WooCommerce\Admin\BlockTemplates\BlockTemplateInterface` interface.
+
+**A template should be instantiated in or after an `init` action hook, priority 4 or higher.**
+
+## Block addition and removal
+
+After a template instance is created, blocks can be added to or removed from a template using the `add_block()` and `remove_block()` methods, or similar methods that are specific to the type of block being added or removed, such as `add_section()` and `remove_section()`.
+
+Blocks can be added or removed immediately after instantiation.
+
+See the [Automattic\WooCommerce\Admin\BlockTemplates](../../plugins/woocommerce/src/Admin/BlockTemplates/README.md) documentation for more information about these methods.
+
+### Actions
+
+The following actions are fired when blocks are added to or removed from a template, to support extensibility:
+
+-  `woocommerce_product_editor_block_template_{template_name}_after_add_block_{block_id}`
+-  `woocommerce_product_editor_block_template_after_add_block`
+-  `woocommerce_product_editor_block_template_{template_name}_after_remove_block_{block_id}`
+-  `woocommerce_product_editor_block_template_after_remove_block`
+
+**In order for your action hooks to be called for all block additions and removals for a template, you should call `add_action()` for each of these hooks before the template is instantiated, in or before an `init` action hook, priority 3 or lower.**
+
+See the [Automattic\WooCommerce\Admin\BlockTemplates](../../plugins/woocommerce/src/Admin/BlockTemplates/README.md) documentation for more information about these hooks.
+
+## Registration
+
+After a template is instantiated, it can be registered with the `Automattic\WooCommerce\Internal\Admin\BlockTemplateRegistry\BlockTemplateRegistry`.
+
+Registration is required in order for the template to be sent to the client. 
+
+Blocks can be added or removed from a template before or after it is registered, but the template cannot be modified after it is sent to the client.
+
+**In order for the template to be sent to the client, it should be in or before the `admin_enqueue_scripts` action hook, priority 9 or lower.**
+
+## Sent to client
+
+A template is sent to the client in or after the `admin_enqueue_scripts` action hook, priority 10 or higher.
+
+Any template modification after this point will not be sent to the client.
+
+## Rendered on client
+
+When the template is rendered on the client, all blocks in the template have their `hideConditions` evaluated to determine whether they should be rendered or not.