mdbx-docs: add Improvements beyond LMDB to the main TOC.

docs/_toc.md

@@ -1,3 +1,12 @@
+### We have migrated to a reliable trusted infrastructure
+
+The origin for now is at
+[GitFlic](https://gitflic.ru/project/erthink/libmdbx) since on
+2022-04-15 the Github administration, without any warning nor
+explanation, deleted _libmdbx_ along with a lot of other projects,
+simultaneously blocking access for many developers. For the same reason
+~~Github~~ is blacklisted forever.
+
 _The Future will (be) [Positive](https://www.ptsecurity.com). Всё будет хорошо._
 
 \section toc Table of Contents
@@ -7,6 +16,7 @@ each of which is divided into several sections.
 
 1. The \ref intro
   - \ref characteristics
+  - \ref improvements
   - \ref restrictions
   - \ref performance
 2. \ref usage