Describe new compression method API in documentation

Add to https://openexr.com/en/latest/ReadingAndWritingImageFiles.html#compression, source is [here](https://github.com/AcademySoftwareFoundation/openexr/blob/main/website/ReadingAndWritingImageFiles.rst#compression).

In anticipation of future support for new compression types, there is now a convenience API for mapping between compression type names and the associated enum:

```
getCompressionDescriptionFromId(Compression, std::string&)
getCompressionIdFromName(const std::string&, Compression&)
getCompressionNameFromId(Compression, std::string&)
getCompressionNamesString(const std::string&, std::string&)
getCompressionNumScanlines(Compression)
isValidCompression(int)
```

Open website/ReadingAndWritingImageFiles.rst and locate the compression section linked by the issue. Compare it with the listed compression convenience API functions and document their purpose and signatures there. Done means the rendered Reading and Writing Image Files compression section describes all six functions.

Written by the indexing model from the issue text.