Restructure explainers (#302)

- Revives the legacy Phase 3 explainer and put it in
  `proposals/exception-handling/legacy` directory. Given that we still
  maintain the legacy formal spec in `document/legacy`, I think it makes
  sense to have the legacy expliner too.
- Moves two auxiliary docs for the legacy proposal
  (`Exception-formal-overview.md` and `Exception-formal-examples.md`),
  currently still in `proposals/exception-handling`, to the new
  `proposals/exception-handling/legacy` directory too.
- Renames `proposals/exception-handling/old` to
  `proposals/exception-handling/pre-legacy`, signifying this precedes
  the "legacy" proposal.
- Fixes the CG meeting date from the current explainer.
- Adds more historical context to the beginning of each explainer in
  `pre-legacy` and `legacy` directories.

proposals/exception-handling/Exceptions.md

@@ -1,8 +1,8 @@
 # Exception handling
 
 This explainer reflects the up-to-date version of the exception handling
-proposal agreed on [the CG meeting on
-09-15-2020](https://github.com/WebAssembly/meetings/blob/master/main/2020/CG-09-15.md).
+proposal agreed on [Oct 2023 CG
+meeting](https://github.com/WebAssembly/meetings/blob/main/main/2023/CG-10.md#exception-handling-vote-on-proposal-to-re-introduce-exnref).
 
 ---