Configure: DOXYGEN for documentation generation (#4701)

* Update doxygen.cfg

* Create p4c_header.html

- Allows for custom header

* Fix: Relative paths to resources

* Git Ignore : Doxygen Warning logs

* Doc: Update P4 logo

* Update : Footer and CSS file for Doxygen v1.9.1

* Bump up Doxygen v1.9.1 -> v1.11.0(latest)

Please review the Doxygen 1.11.0 release notes for detailed information on changes:

https://www.doxygen.nl/manual/changelog.html#log_1_11

.gitignore

@@ -45,3 +45,4 @@ bazel-*
 
 # JetBrains
 .idea/
+docs/doxygen/p4c-doxygen-warn.log