Fix dead links and menu entries

[webmaniak]

Hello,

This is my first contribution to the IPFS documentation, so better start small 😄

1. I noticed a few links were dead in the menu, I fixed them
2. I also noticed there was no link to the ipfs/docs repo, I added it for convenience

I hope this contribution will be accepted, it's rather simple but remarks and constructive comments are always welcome!

[daviddias]

Thank you ! :)

[webmaniak]

@daviddias your welcome, it was my pleasure to make that small contribution. Please tell me when there would be additional changes you'd like to see.

[daviddias]

@webmaniak if you are interested, take a peek at our sea of issues -- https://waffle.io/ipfs/ipfs -- and search for the help wanted ones :) (depending on your connection, it might take it a minute or 2 in first load)