Improve performance of creating documentation models

[SUPERCILEX]

We're investigating performance and found the bulk of the work is spent creating documentation models:

                           Initializing plugins:   111
                  Creating documentation models: 93410
Transforming documentation model before merging:   351
                   Merging documentation models:    25
 Transforming documentation model after merging:  1688
                                 Creating pages:     3
                             Transforming pages:    48
                                      Rendering:  5552

Unfortunately, this isn't because there's a lot of work to do, but rather because it isn't parallelized. Monitoring the cores shows that only one is used at a time.

I believe the DefaultPsiToDocumentableTranslator and DefaultDescriptorToDocumentableTranslator can be parallelized. For example, parsing each class should be able to be done independently.

[kamildoleglo]

Duplicate of #1212