Update documentation and arduino debug options (#8837)

* Update documentation

Describe how to improve Exception Decoder results.

Updated build option details.

* Added d-a-v's code. And updated description and debug macro.

* Update doc

Updated platform.txt - positioned -Os such that it can be overridden by `build.opt`.

Add suggestion of using `-Og` which may improve the Decoder's line number correlation
with the problem in the source code.

Untested adjustments to platformio-build.py

* Fixed code formatting

* Fixed example

Previously `-O3` in the example had no effect. It was overridden by later flags.
Reording the `-Os` allowed changing optimization to  work. Somehow this triggered
a new warning "Stream.h:50:7: warning: 'f.Stream::_startMillis' may be used
uninitialized in this function". Replaced `-O3` with `-Og`.

* Proposed "Debug port" menu change
Preserves a pathway for `build.opt` override.

* Update boards.txt.py and docs - WIP

* Improve organization of optimization content.
Use "Debug Optimization" for menu name.
The menu item defines the initial optimization.
platform.txt - optimization parameter can be overridden by build.opt.

* Add fallback value for build.debug_optim to platform.txt

* update text and undo changes to platformio-build.py

* tweak text

* Added ':orphan:' mark to a06-global-build-options.rst

* Update doc. Added link in page index to 'Improving Exception Decoder Results'

* Update text to reference PR#8868 improvements for leaf function.

cores/esp8266/debug.h

@@ -4,8 +4,13 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#define _DEBUG_LEAF_FUNCTION(...) __asm__ __volatile__("" ::: "a0", "memory")
+
 #ifdef DEBUG_ESP_CORE
 #define DEBUGV(fmt, ...) ::printf((PGM_P)PSTR(fmt), ##__VA_ARGS__)
+#define DEBUG_LEAF_FUNCTION _DEBUG_LEAF_FUNCTION
+#else
+#define DEBUG_LEAF_FUNCTION(...)
 #endif
 
 #ifndef DEBUGV