[ViktorBystrov72]

Document that map literals create map[string]any, including conversion of numeric keys to strings. Show string-key lookup and membership examples, and distinguish literals from Go maps supplied through the environment, which retain their declared key types.

Closes #862. Documentation only.

Validation:

• Reproduced both reported integer-key errors and exercised the documented literal and map[int]string forms.
• go test ./checker -run '^TestCheck($|_error$|_operator_in_works_with_interfaces$)' -count=1
• go test . -run '^(TestExpr_map_|TestCompile_allow_to_use_interface_to_get_an_element_from_map|TestExpr_env_types_map|TestIssue758_filter_map_index|ExampleEnv$)' -count=1
• git diff --check