HTJ2K Compressor Documentation introduction

[TodicaIonut]

Website preview: https://openexr--2052.org.readthedocs.build/en/2052/

[peterhillman]

This should state that HTJ2K is not yet in an official release. Once it is released, it should state the minimum version required to read files with HTJ2K compression. (Or we delay updating the Technical Introduction until after the next release)