add first draft of API Lifecycle docs (#6229)

* first attempt at lifecycle sequence docs

closes #6197

* address Kyle's review

I hope this is right...

* address Shannon's comments

address Shannon's comments

Author: tsiq-swyx

docs/docs/gatsby-lifecycle-apis.md

@@ -13,4 +13,49 @@ Gatsby's design principles include:
   trivial and encouraged.
 - Plugins are easy to open source and reuse. They're just NPM packages.
 
-See the links along the left under "REFERENCE" for the full API documentation.
+# High level Overview
+
+## Bootstrap sequence
+
+During "bootstrap" gatsby:
+
+- reads `gatsby-config.js` to load in your list of plugins
+- initializes its cache (stored in `/.cache`)
+- pulls in and preprocesses data ("source and transform nodes") into a GraphQL schema
+- creates pages in memory
+  - from your `/pages` folder 
+  - from your `gatsby-node.js` if you implement `createPages`/`createPagesStatefully` (e.g. templates)
+  - from any plugins that implement `createPages`/`createPagesStatefully`
+- extracts, runs, and replaces graphql queries for pages and `StaticQuery`s
+- writes out the pages to cache
+
+In development this is a running process powered by [Webpack](https://github.com/gatsbyjs/gatsby/blob/dd91b8dceb3b8a20820b15acae36529799217ae4/packages/gatsby/package.json#L128) and [`react-hot-loader`](https://github.com/gatsbyjs/gatsby/blob/dd91b8dceb3b8a20820b15acae36529799217ae4/packages/gatsby/package.json#L104), so changes to any files get re-run through the sequence again, with [smart cache invalidation](https://github.com/gatsbyjs/gatsby/blob/ffd8b2d691c995c760fe380769852bcdb26a2278/packages/gatsby/src/bootstrap/index.js#L141). For example, `gatsby-source-filesystem` watches files for changes, and each change triggers re-running queries. Other plugins may also perform this service. Queries are also watched so if you modify a query your development app is hot reloaded.
+
+The core of the bootstrap process is the "api-runner", which helps to execute APIs in sequence, with state managed in Redux. Gatsby exposes a number of lifecycle APIs which can either be implemented by you (or any of your configured plugins) in `gatsby-node.js`, `gatsby-browser.js` or `gatsby-ssr.js`.
+
+The sequence of the **main** bootstrap Node API lifecycles are:
+
+- [onPreBootstrap](https://www.gatsbyjs.org/docs/node-apis/#onPreBootstrap) e.g. implemented by [`gatsby-plugin-typography`](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-typography/src/gatsby-node.js)
+- [sourceNodes](https://www.gatsbyjs.org/docs/node-apis/#sourceNodes) e.g. implemented by [`gatsby-source-wikipedia`](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-wikipedia/src/gatsby-node.js)
+  - within this `createNode` can be called multiple times, which then triggers [onCreateNode](https://www.gatsbyjs.org/docs/node-apis/#onCreateNode).
+- (the first schema build happens here)
+- [resolvableExtensions](https://www.gatsbyjs.org/docs/node-apis/#resolvableExtensions) for filetype/language extensions eg [`gatsby-plugin-typescript`](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-typescript/src/gatsby-node.js)
+- [createPages](https://www.gatsbyjs.org/docs/node-apis/#createPages) e.g. implemented by [`page-hot-reloader`](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/bootstrap/page-hot-reloader.js)
+  - within this `createPage` can be called any number of times, which then triggers [onCreatePage](https://www.gatsbyjs.org/docs/node-apis/#onCreatePage)
+- [onPreExtractQueries](https://www.gatsbyjs.org/docs/node-apis/#onPreExtractQueries) e.g. implemented by [`gatsby-transformer-sharp`](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-sharp/src/gatsby-node.js) and [`gatsby-source-contentful`](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-contentful/src/gatsby-node.js)
+- (schema update happens here)
+- **extract queries from components** where the [queryCompiler](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/internal-plugins/query-runner/query-compiler.js#L189) replaces page GraphQL queries and `StaticQueries`
+- The [queries are run](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/internal-plugins/query-runner/page-query-runner.js#L100), and the [pages are written out](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/internal-plugins/query-runner/pages-writer.js)
+- [onPostBoostrap](https://www.gatsbyjs.org/docs/node-apis/#onPostBootstrap) is called (but it is not often used)
+
+## Build sequence
+
+(to be written)
+
+## Client sequence
+
+(to be written)
+
+---
+
+Please see the links along the left under "REFERENCE" for the full API documentation.