Add plugins explanation doc

docs/explanations/plugins.md

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+# Plugins
+
+We can think about Scanio as about 2 major components:
+1. Scanio Core
+2. Scanio Plugins
+
+The Scanio is built with a plugin system in mind. All companies have a unique setup, use different tools. Plugin system helps to unify the experience of using scanio, make it easy to switch between different plugins, and give a power of extensibility by impleneting new plugins. The main function of plugins is to glue core with different systems and hide the complexities of individual systems behind a single interface.
+The Scanio Core is focused on implementing more of common code, command handling, input validation, logging, different utility functionalities. For integrations with other systems it invokes dedicated plugins.
+
+Currently there are 2 main groups of plugins incorporated into scanio:
+1. **Scanner Plugins**: These plugins integrate with security tools to perform scans. Examples include:
+   - **[Bandit Plugin](docs/reference/plugin-bandit.md)**: Integrates with the Bandit scanner for analyzing Python code.
+   - **[Semgrep Plugin](docs/reference/plugin-semgrep.md)**: Enables static analysis using Semgrep.
+   - **[Trufflehog3 Plugin](docs/reference/plugin-trufflehog3.md)**: Scans for secrets in codebases.
+2. **VCS Plugins**: These plugins interact with version control systems (VCS) to list repositories, clone code, or manage pull requests. Examples include:
+   - **[GitLab Plugin](docs/reference/plugin-gitlab.md)**: Works with GitLab repositories.
+   - **[GitHub Plugin](docs/reference/plugin-github.md)**: Integrates with GitHub for repository management.
+   - **[Bitbucket Plugin](docs/reference/plugin-bitbucket.md)**: Facilitates interactions with Bitbucket.
+
+You can read more about each individual plugins on their dedicated reference doc pages.
+
+## Implementation
+The plugins system is built with the help of `hashicorp/go-plugin` module. It manages the trigger of plugins execution, connection and function execution over RCP, and other. On scanio side connection interfaces were implemented.  
+Interface for VCS plugins can be found in [pkg/shared/ivcs.go](pkg/shared/ivcs.go) defines the interface of plugins. Each plugin must define all of the function described in the `type VCS interface`. Interface file also defines all parameter types, needed for plugins function call.  
+Same as for vcs plugins, there is a file that defines interface and related types for scanners in [iscanner.go](/pkg/shared/iscanner.go).
+There is no plugins registration mechanism. Plugins are rather invoked by known filesystem path to the plugin, and a handshake mechanism.
+
+
+### Plugin implementation notes
+The plugin side of Scanio is where the actual functionality for individual scanners is implemented.
+
+To start serving as a plugin, the `plugin.Serve` method should be called. It accepts several parameters: a shared handshake config, a logger, and a plugin defined by the plugin itself.
+```go
+plugin.Serve(&plugin.ServeConfig{
+  HandshakeConfig: shared.HandshakeConfig,
+  Plugins: map[string]plugin.Plugin{
+    shared.PluginTypeScanner: &shared.ScannerPlugin{Impl: scannerInstance},
+  },
+  Logger: logger,
+})
+```
+
+The `scannerInstance` structure must implement all method defined by the plugin interface definition. For scanner plugin it's `Setup` and `Scan` methods. A plugin may also contain other methods.
+```go
+type Scanner interface {
+	Setup(configData config.Config) (bool, error)
+	Scan(args ScannerScanRequest) (ScannerScanResponse, error)
+}
+```
+
+The main purpose of `Setup` function is to pass the config file data, that can be later used by the plugin to handle main functionality.