Change idris-lang.org to www.idris-lang.org in README and docs

[Z-snails]

The DNS records/github pages config for idris-lang.org seems to be wrong, so the wrong TLS certificate is served for idris-lang.org. Using www.idris-lang.org avoids this issue. Also www.idris-lang.org is the canonical domain, since idris-lang.org redirects to www.idris-lang.org.

The ideal solution would be to fix idris-lang.org, but I don't know how to do that, so in the mean time this will help avoid some confusion for new-comers.

Self-check

[dunhamsteve]

I believe both www.idris-lang.org and idris-lang.org would be handled if the github config were changed to say idris-lang.org instead of www.idris-lang.org. (Based on this github documentation)

Also, to get ip6 working the AAAA records for idris-lang.org at mythic-beasts.org should be changed to:

2606:50c0:8000::153
2606:50c0:8001::153
2606:50c0:8002::153
2606:50c0:8003::153
```
(from the documentation at the above link).  Currently it is pointed at a mythic-beasts address.