doc/md: document project files (#810)

* doc/md: document project files

* h3

* Update doc/md/cli/projects.md

Co-authored-by: Ariel Mashraki <[email protected]>

* Update doc/md/cli/projects.md

Co-authored-by: Ariel Mashraki <[email protected]>

* Update doc/md/cli/projects.md

Co-authored-by: Ariel Mashraki <[email protected]>

* Update doc/md/cli/projects.md

Co-authored-by: Ariel Mashraki <[email protected]>

Co-authored-by: Ariel Mashraki <[email protected]>

doc/md/cli/projects.md

@@ -0,0 +1,124 @@
+---
+title: Project Structure
+id: projects
+slug: /cli/projects
+---
+### Project Files
+
+Project files provide a convenient way to describe and interact with multiple
+environments when working with Atlas. A project file is a file named
+`atlas.hcl` and contains one or more `env` blocks. For example:
+
+```hcl
+// Define an environment named "local"
+env "local" {
+  // Declare where the schema definition file resides.
+  src = "./schema/project.hcl"
+  
+  // Define the URL of the database which is managed in
+  // this environment.
+  url = "mysql://localhost:3306"
+  
+  // Define the URL of the Dev Database for this environment
+  // See: https://atlasgo.io/dev-database
+  dev = "mysql://localhost:3307"
+  
+  // The schemas in the database that are managed by Atlas.
+  schemas = ["users", "admin"]
+}
+
+env "dev" {
+  // ... a different env
+}
+```
+
+Once defined, a project's environment can be worked against using the `--env` flag. 
+For example:
+
+```shell
+atlas schema apply --env local
+```
+
+Will run the `schema apply` command against the database that is defined for the `local`
+environment. 
+
+### Projects with Versioned Migrations
+
+Environments may declare a `migration_dir` block to configure how versioned migrations
+work in the specific environment:
+
+```hcl
+env "local" {
+    // ..
+    migration_dir {
+        // URL where the migration directory resides. Only filesystem directories
+        // are currently supported but more options will be added in the future.
+        url = "file://migrations"
+        // Format of the migration directory: atlas | flyway | liquibase | goose | golang-migrate
+        format = atlas
+    }
+}
+```
+
+Once defined, `migrate` commands can use this configuration, for example:
+```shell
+$ atlas migrate validate --env local
+```
+Will run the `migrate validate` command against the Dev Database defined in the 
+`local` environment. 
+
+### Passing Input Values
+
+Project files may pass [input values](/ddl/input-variables) to variables defined in 
+the Atlas schema of the environment. To do this simply provide additional attributes
+to the environment block:
+```hcl
+env "local" {
+  url = "sqlite://test?mode=memory&_fk=1"
+  src = "./schema.hcl"
+  
+  // Other attributes are passed as input values to "schema.hcl":
+  tenant = "rotemtam"
+}
+```
+
+These values can then be consumed by variables defined in the schema file:
+
+```hcl
+variable "tenant" {
+  type = string
+}
+schema "main" {
+  name = var.tenant
+}
+```
+
+### Project Input Variables
+
+Project files may also declare [input variables](../ddl/input.md) that can be supplied to the CLI 
+at runtime. For example: 
+
+```hcl
+variable "tenant" {
+  type = string
+}
+
+env "local" {
+  url = "sqlite://test?mode=memory&_fk=1"
+  src = "./schema.hcl"
+  
+  // The value for "tenant" is determined by the user at runtime.
+  tenant = var.tenant
+}
+```
+To set the value for this variable at runtime, use the `--var` flag:
+
+```shell
+$ atlas schema apply --env local --var tenant=rotemtam
+```
+
+It is worth mentioning that when running Atlas commands within a project using
+the `--env` flag, all input values supplied at the command-line are passed only
+to the project file, and not propagated automatically to children schema files. 
+This is done with the purpose of creating an explicit contract between the environment
+and the schema file.

doc/website/sidebars.js

@@ -43,6 +43,7 @@ module.exports = {
             items: [
                 {type: 'doc', id: 'cli/cli-reference', label: 'Reference'},
                 {type: 'doc', id: 'cli/cli-url', label: 'URLs'},
+                {type: 'doc', id: 'cli/projects'},
             ]
         },
         {