feat: added javadocs

Description

feat: added javadocs

• enabled checkstyle javadoc checks
• added missing and updated existing javadocs

How to test

No need

Author's checklist

• I have followed the contribution guidelines
• This MR has been tested or does not change functionality
• I have added relevant merge request dependencies (if this MR has any)
• I have added the correct labels
• I have assigned reviewers (if any are relevant)
• I have edited the documentation (if the changes require it) or I have noted the need for the change if I do not have access to the documentation
• I have marked all introduced BREAKING CHANGES or necessary DEPLOYMENT NOTES in the commit message(s)

Reviewer's checklist

• This MR has been tested or does not change functionality
• This MR has correct commit message format

Other information

I was not quite sure what a small number of classes did (for example RegistrarConfig), tried to improvise and piece information together from https://is.muni.cz/auth/th/or7qs/thesis.pdf

Related issues

Closes STR-1195