Updating NGT Corpus details

[cleong110]

fixes https://github.com/sign-language-processing/sign-language-processing.github.io/issues/58

Q: where did I get these numbers? A: described my process in the issue but tl;dr:

• Sign count: counted manually with the data loader
• new link: used new official site
• Various other figures e.g. signer count, session count: looked at official site, dataloader, official citation, tried to judge what was now accurate.