Update links to tesseract documentation

Several links present in the README pointing to the tesseract documentation are outdated.

https://github.com/tesseract-ocr/tesseract/wiki
> These wiki pages are no longer maintained. All pages were moved to tesseract-ocr/tessdoc.
The latest documentation is available at https://tesseract-ocr.github.io/.

Update links to https://github.com/tesseract-ocr/tessdoc/