Docs: reading and writing files; patch_inplace() is not atomic

- New page features/files.md: read a file with a stream and check that it
  was opened, serialize before opening the output file, replace a file via a
  temporary file and rename, binary formats in binary mode. The library keeps
  no file I/O of its own.
- patch_inplace(): say what a failed patch leaves behind (the earlier
  operations, and a failing move may already have removed its source) and
  point to patch() for all-or-nothing behaviour.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/mkdocs.yml

@@ -130,6 +130,7 @@ nav:
           - features/element_access/unchecked_access.md
           - features/element_access/checked_access.md
           - features/element_access/default_value.md
+      - features/files.md
       - features/iterators.md
       - features/merge_patch.md
       - features/json_patch.md