[Comgr][NFC] Create level 3 heading for env vars

Author: lamb-j

amd/comgr/README.md

@@ -152,6 +152,7 @@ Comgr supports an environment variable to help locate LLVM:
   installation, which is currently used to locate the clang resource directory
   and clang binary path, allowing for additional optimizations.
 
+### Caching
 Comgr utilizes a cache to preserve the results of compilations between executions.
 The cache's status (enabled/disabled), storage location for its results,
 and eviction policy can be manipulated through specific environment variables.
@@ -172,6 +173,7 @@ By default, the cache is enabled.
   termination. The string format aligns with [Clang's ThinLTO cache pruning policy](https://clang.llvm.org/docs/ThinLTO.html#cache-pruning).
   The default policy is set as: "prune_interval=1h:prune_expiration=0h:cache_size=75%:cache_size_bytes=30g:cache_size_files=0".
 
+### Debugging
 Comgr supports some environment variables to aid in debugging. These
 include:
 
@@ -192,6 +194,7 @@ include:
 * `AMD_COMGR_TIME_STATISTICS`: If this is set, and is not "0", logs will
   include additional Comgr-specific timing information for compilation actions.
 
+### VFS
 Comgr implements support for an in-memory, virtual filesystem (VFS) for storing
 temporaries generated during intermediate compilation steps. This is aimed at 
 improving performance by reducing on-disk file I/O. Currently, VFS is only supported