What is total_allocations?

What is [total allocations](https://github.com/bloomberg/memray/blob/f2c180db9e9ff32d03b100a7a4ac086c39eb0367/src/memray/_metadata.py#L15)?

For whatever reason, this only shows up in the JSON report (see #620).

Suppose I get an output like this:

```
{'total_num_allocations': 2844284,
'total_bytes_allocated': 526180830,
'allocation_size_histogram': [...],
'allocator_type_distribution': {'PYMALLOC_MALLOC': 2721820,
'PYMALLOC_REALLOC': 122350,
'REALLOC': 86,
'PYMALLOC_CALLOC': 22,
'MALLOC': 6},
'top_allocations_by_size': [...],
'metadata': {
'total_allocations': 5686611,
'total_frames': 46,
'peak_memory': 4073520,
'python_allocator': 'pymalloc',
'has_native_traces': False,
'trace_python_allocators': True,
'file_format': 0}}
```

Notice that `total_num_allocations` doesn't match `total_allocations`. It would be nice in `memray` included somewhere on its website a description of:

- what `total_num_allocations` is
- what `total_allocations` is
- what `total_frames` is
- what precisely `peak_memory` is

Start with src/memray/_metadata.py and the JSON report behavior referenced in #620. Trace the meanings of total_num_allocations, total_allocations, total_frames, and peak_memory, then document each on the Memray website, including why the allocation counts differ.

Written by the indexing model from the issue text.