Add Doxygen welcome page with organized navigation

Creates a main landing page for the API documentation with:
- Quick links to core classes (BehaviorTreeFactory, Blackboard, TreeNode)
- Node Types overview table
- Complete list of built-in Control Nodes (Sequences, Fallbacks, Parallels, Conditional)
- Complete list of built-in Decorators (Repetition, Timing, Result Modification, etc.)
- Complete list of built-in Actions (Status, Blackboard, Utility, Scripting, Queue)
- Core Concepts, Logging & Tools, and external Resources

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide

Doxyfile

@@ -781,7 +781,8 @@ WARN_LOGFILE           =
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = ./include
+INPUT                  = ./include \
+                         ./docs/mainpage.md
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses
@@ -974,7 +975,7 @@ FILTER_SOURCE_PATTERNS =
 # (index.html). This can be useful if you have a project on for instance GitHub
 # and want to reuse the introduction page also for the doxygen output.
 
-USE_MDFILE_AS_MAINPAGE =
+USE_MDFILE_AS_MAINPAGE = docs/mainpage.md
 
 #---------------------------------------------------------------------------
 # Configuration options related to source browsing
@@ -1184,7 +1185,8 @@ HTML_STYLESHEET        =
 # list). For an example see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_EXTRA_STYLESHEET  =
+HTML_EXTRA_STYLESHEET  = 3rdparty/doxygen-awesome-css/doxygen-awesome.css \
+                         3rdparty/doxygen-awesome-css/doxygen-awesome-sidebar-only.css
 
 # The HTML_EXTRA_FILES tag can be used to specify one or more extra images or
 # other source files which should be copied to the HTML output directory. Note

docs/mainpage.md

@@ -0,0 +1,104 @@
+# BehaviorTree.CPP {#mainpage}
+
+C++ library for behavior tree execution.
+
+## Quick Start
+- @ref BT::BehaviorTreeFactory - Create and register nodes
+- @ref BT::Blackboard - Shared data storage
+- @ref BT::TreeNode - Base class for all nodes
+
+## Node Types
+
+| Category | Description | Key Classes |
+|----------|-------------|-------------|
+| **Actions** | Execute tasks | @ref BT::SyncActionNode, @ref BT::StatefulActionNode |
+| **Conditions** | Check state | @ref BT::ConditionNode |
+| **Control** | Flow control | @ref BT::SequenceNode, @ref BT::FallbackNode, @ref BT::ParallelNode |
+| **Decorators** | Modify behavior | @ref BT::RetryNode, @ref BT::TimeoutNode, @ref BT::InverterNode |
+
+## Built-in Control Nodes
+
+### Sequences
+- @ref BT::SequenceNode
+- @ref BT::ReactiveSequence
+- @ref BT::SequenceWithMemory
+
+### Fallbacks
+- @ref BT::FallbackNode
+- @ref BT::ReactiveFallback
+
+### Parallels
+- @ref BT::ParallelNode
+- @ref BT::ParallelAllNode
+
+### Conditional
+- @ref BT::IfThenElseNode
+- @ref BT::WhileDoElseNode
+- @ref BT::SwitchNode
+- @ref BT::ManualSelectorNode
+
+## Built-in Decorators
+
+### Repetition
+- @ref BT::RetryNode
+- @ref BT::RepeatNode
+- @ref BT::LoopNode
+
+### Timing
+- @ref BT::TimeoutNode
+- @ref BT::DelayNode
+
+### Result Modification
+- @ref BT::InverterNode
+- @ref BT::ForceSuccessNode
+- @ref BT::ForceFailureNode
+
+### Execution Control
+- @ref BT::RunOnceNode
+- @ref BT::KeepRunningUntilFailureNode
+
+### Subtrees
+- @ref BT::SubTreeNode
+
+### Preconditions
+- @ref BT::PreconditionNode
+- @ref BT::EntryUpdatedDecorator
+
+## Built-in Actions
+
+### Status
+- @ref BT::AlwaysSuccessNode
+- @ref BT::AlwaysFailureNode
+
+### Blackboard
+- @ref BT::SetBlackboardNode
+- @ref BT::UnsetBlackboardNode
+
+### Utility
+- @ref BT::SleepNode
+- @ref BT::TestNode
+
+### Scripting
+- @ref BT::ScriptNode
+- @ref BT::ScriptCondition
+
+### Entry Updated
+- @ref BT::EntryUpdatedAction
+
+### Queue
+- @ref BT::PopFromQueue
+- @ref BT::QueueSize
+
+## Core Concepts
+- @ref BT::PortInfo - Type-safe port system
+- @ref BT::Expected - Result type for error handling
+- @ref BT::NodeStatus - Node execution states
+
+## Logging & Tools
+- @ref BT::FileLogger2 - File logging
+- @ref BT::StdCoutLogger - Console output
+- @ref BT::Groot2Publisher - Groot2 editor integration
+
+## Resources
+- [GitHub Repository](https://github.com/BehaviorTree/BehaviorTree.CPP)
+- [Groot2 Editor](https://www.behaviortree.dev/)