mdbx-doc: provide non-API docs via doxygen (squashed).

Change-Id: Ie33858517f964f794ec182a1e8bb630730a0f172

docs/_toc.md

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+
+_The Future will (be) [Positive](https://www.ptsecurity.com). Всё будет хорошо._
+
+\section toc Table of Contents
+
+This manual is divided into parts,
+each of which is divided into several sections.
+
+1. The \ref intro
+  - \ref characteristics
+    - Preface
+    - Features
+    - Limitations
+    - Gotchas
+    - Comparison with other databases
+  - \ref restrictions
+  - \ref performance
+    - Integral performance
+    - Read Scalability
+    - Sync-write mode
+    - Lazy-write mode
+    - Async-write mode
+    - Cost comparison
+2. \ref usage
+  - \ref getting
+    - Embedding
+    - Building
+  - \ref starting
+    - Opening
+    - Cursors
+    - Threads and processes
+    - Transactions
+    - Duplicate keys aka Multi-values
+    - Cleaning up
+  - \ref bindings
+
+3. The `C` API reference manual:
+  - TODO
+
+Please do not hesitate to point out errors in the documentation,
+including creating [PR](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/proposing-changes-to-your-work-with-pull-requests) with corrections and improvements.
+
+---
+
+\section mithril Mithril DB