doc: Rename directory_structure.md to overview.md

Change title, move to Concepts section. Otherwise
the document remains the same.

Change-Id: I15dfe541447a620e270d94df32c76b36082ef4be
Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/453451
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>

doc/Doxyfile

@@ -810,7 +810,6 @@ INPUT                  += \
                          changelog.md \
                          compression.md \
                          concurrency.md \
-                         directory_structure.md \
                          event.md \
                          ftl.md \
                          gdb_macros.md \
@@ -827,6 +826,7 @@ INPUT                  += \
                          nvmf.md \
                          nvmf_tgt_pg.md \
                          nvmf_tracing.md \
+                         overview.md \
                          peer_2_peer.md \
                          porting.md \
                          spdkcli.md \

doc/concepts.md

@@ -5,4 +5,5 @@
 - @subpage concurrency
 - @subpage ssd_internals
 - @subpage vhost_processing
+- @subpage overview
 - @subpage porting

doc/general.md

@@ -1,6 +1,5 @@
 # General Information {#general}
 
-- @subpage directory_structure
 - @subpage event
 - @subpage logical_volumes
 - @subpage vpp_integration

doc/overview.md

@@ -1,4 +1,4 @@
-# SPDK Directory Structure {#directory_structure}
+# SPDK Structural Overview {#overview}
 
 # Overview {#dir_overview}
 

doc/ssd_internals.md

@@ -1,4 +1,4 @@
-# SSD Internals {#ssd_internals}
+# NAND Flash SSD Internals {#ssd_internals}
 
 Solid State Devices (SSD) are complex devices and their performance depends on
 how they're used. The following description is intended to help software