[ruccho]

This PR adds a guide that explains the cost of recording and how the encoding options affect it, in English (docs/performance-guide.md) and Japanese (docs/performance-guide.ja.md). The README links to it from the section on memory usage.

The guide covers the following:

• Where memory is used during recording: the capture texture, the encoder, the buffer of encoded frames and the native allocator.
• The CPU and GPU load of the blit and readback paths on iOS and Android.
• The cost of creating a session and exporting.
• The effect of the encoding size, the frame rate, the bitrate, the memory limit, the disk buffer and ForceReadback.
• Recommendations based on these, and the measured values the descriptions are based on.

The guide describes the versions after v1.7.0, including #198, #199, #200 and #201.

• The descriptions of the buffer of encoded frames, the export and the recommendations reflect those changes.
• The tables of measured values were obtained with v1.7.0 on an iPhone 12 and a Galaxy S22, and are labelled as such.
• A section "Changes after v1.7.0" lists the changes and the values measured after them on the Galaxy S22:
  • the managed heap and physical memory with a full buffer;
  • the managed heap after the session is disposed;
  • the longest frame during export;
  • the CPU time of the threads of the native library.
• iOS was not measured again.

The CPU usage described for Android assumes native binaries rebuilt with the changes of #201, which the release procedure requires.