Move plugin-sdk to in-tree in core repository

Moving terraform-docs/plugin-sdk standalone module to in-tree, because
maintaining both of them, specifically if anything needs to be added to
Config, or terraform will required dual effort on both repository. As
such now everything is consolidated under one repository. Example usage
for plugin developer after this move is as follow:

```go
package main

import (
	_ "embed" //nolint

	"github.com/terraform-docs/terraform-docs/plugin"
	"github.com/terraform-docs/terraform-docs/print"
	"github.com/terraform-docs/terraform-docs/template"
	"github.com/terraform-docs/terraform-docs/terraform"
)

func main() {
	plugin.Serve(&plugin.ServeOpts{
		Name:    "template",
		Version: "0.1.0",
		Printer: printer,
	})
}

//go:embed sections.tmpl
var tplCustom []byte

// Print the custom format template. You have all the flexibility to generate
// the output however you choose to.
func printer(config *print.Config, module *terraform.Module) (string, error) {
	tpl := template.New(config,
		&template.Item{Name: "custom", Text: string(tplCustom)},
	)

	rendered, err := tpl.Render("custom", module)
	if err != nil {
		return "", err
	}

	return rendered, nil
}
```

Signed-off-by: Khosrow Moossavi <khos2ow@gmail.com>

README.md

@@ -253,6 +253,14 @@ when formatter is set to `markdown table`.
 Note that sections visibility (i.e. `sections.show` and `sections.hide`) takes
 precedence over the `content`.
 
+Additionally there's also one extra special variable avaialble to the `content`:
+
+- `{{ .Module }}`
+
+As opposed to the other variables mentioned above, which are generated sections
+based on a selected formatter, the `{{ .Module }}` variable is just a `struct`
+representing a [Terraform module].
+
 ````yaml
 content: |-
   Any arbitrary text can be placed anywhere in the content
@@ -278,6 +286,12 @@ content: |-
   ```hcl
   {{ include "examples/foo/main.tf" }}
   ```
+
+  ## Resources
+
+  {{ range .Module.Resources }}
+  - {{ .GetMode }}.{{ .Spec }} ({{ .Position.Filename }}#{{ .Position.Line }})
+  {{- end }}
 ````
 
 ## Build on top of terraform-docs
@@ -320,6 +334,68 @@ func buildTerraformDocs(path string, tmpl string) (string, error) {
 }
 ```
 
+## Plugin
+
+Generated output can be heavily customized with [`content`], but if using that
+is not enough for your use-case, you can write your own plugin.
+
+In order to install a plugin the following steps are needed:
+
+- download the plugin and place it in `~/.tfdocs.d/plugins` (or `./.tfdocs.d/plugins`)
+- make sure the plugin file name is `tfdocs-format-<NAME>`
+- modify [`formatter`] of `.terraform-docs.yml` file to be `<NAME>`
+
+**Important notes:**
+
+- if the plugin file name is different than the example above, terraform-docs won't
+be able to to pick it up nor register it properly
+- you can only use plugin thorough `.terraform-docs.yml` file and it cannot be used
+with CLI arguments
+
+To create a new plugin create a new repository called `tfdocs-format-<NAME>` with
+following `main.go`:
+
+```go
+package main
+
+import (
+    _ "embed" //nolint
+
+    "github.com/terraform-docs/terraform-docs/plugin"
+    "github.com/terraform-docs/terraform-docs/print"
+    "github.com/terraform-docs/terraform-docs/template"
+    "github.com/terraform-docs/terraform-docs/terraform"
+)
+
+func main() {
+    plugin.Serve(&plugin.ServeOpts{
+        Name:    "<NAME>",
+        Version: "0.1.0",
+        Printer: printerFunc,
+    })
+}
+
+//go:embed sections.tmpl
+var tplCustom []byte
+
+// printerFunc the function being executed by the plugin client.
+func printerFunc(config *print.Config, module *terraform.Module) (string, error) {
+    tpl := template.New(config,
+        &template.Item{Name: "custom", Text: string(tplCustom)},
+    )
+
+    rendered, err := tpl.Render("custom", module)
+    if err != nil {
+        return "", err
+    }
+
+    return rendered, nil
+}
+```
+
+Please refer to [tfdocs-format-template] for more details. You can create a new
+repository from it by clicking on `Use this template` button.
+
 ## Documentation
 
 - **Users**
@@ -341,8 +417,10 @@ MIT License - Copyright (c) 2021 The terraform-docs Authors.
 
 [Chocolatey]: https://www.chocolatey.org
 [Config File Reference]: https://terraform-docs.io/user-guide/configuration/
+[`content`]: https://terraform-docs.io/user-guide/configuration/content/
 [Contributing Guide]: CONTRIBUTING.md
 [Formats Guide]: https://terraform-docs.io/reference/terraform-docs/
+[`formatter`]: https://terraform-docs.io/user-guide/configuration/formatter/
 [here]: https://golang.org/doc/code.html#GOPATH
 [Homebrew]: https://brew.sh
 [install pre-commit]: https://pre-commit.com/#install
@@ -351,5 +429,7 @@ MIT License - Copyright (c) 2021 The terraform-docs Authors.
 [Scoop]: https://scoop.sh/
 [Slack]: https://slack.terraform-docs.io/
 [terraform-docs GitHub Action]: https://github.com/terraform-docs/gh-actions
+[Terraform module]: https://pkg.go.dev/github.com/terraform-docs/terraform-docs/terraform#Module
+[tfdocs-format-template]: https://github.com/terraform-docs/tfdocs-format-template
 [our website]: https://terraform-docs.io/
 [User Guide]: https://terraform-docs.io/user-guide/introduction/