Add env var `TORRUST_INDEX_API_CORS_PERMISSIVE` to docs

[josecelano]

Relates to: torrust/torrust-index-gui#705 (comment)

The Index crate docs don't mention the evn var TORRUST_INDEX_API_CORS_PERMISSIVE which is mantadory to run the Index in development environments.

Besides, the link to the docs in the README is broken:

https://docs.rs/torrust-index/latest/torrust_index/web/api/v1/