Document insert() and erase() of editable json_documents

Add API reference pages for basic_json_document::insert and
basic_json_document::erase, matching the style of set.md and
push_back.md: signatures, parameters, return values, exception safety,
exceptions with their exact ids and messages, complexity, notes on
duplicate keys and view/iterator validity, and an example.

Add example programs basic_json_document__insert.cpp and
basic_json_document__erase.cpp with their expected output, each
comparing an edit on an editable document with the same edit on a
plain json value to show what is preserved: member order, the
spelling of untouched numbers, and, for insert, that a view taken
before the insert keeps referring to the same element after its index
shifts.

Register both new pages in mkdocs.yml, docSet.sql, and the member list
of basic_json_document/index.md, add cross-references to them from
set.md and push_back.md, and mention insert/erase in the "Editing a
document" section of features/json_view.md.

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

docs/mkdocs/mkdocs.yml

@@ -233,6 +233,8 @@ nav:
           - 'Overview': api/basic_json_document/index.md
           - '(Constructor)': api/basic_json_document/basic_json_document.md
           - 'accept': api/basic_json_document/accept.md
+          - 'erase': api/basic_json_document/erase.md
+          - 'insert': api/basic_json_document/insert.md
           - 'is_discarded': api/basic_json_document/is_discarded.md
           - 'memory_usage': api/basic_json_document/memory_usage.md
           - 'node_count': api/basic_json_document/node_count.md