Update documentation to reflect change in intrasite link syntax

Updates getpelican/pelican#1061
Add INTRASITE_LINK_REGEX to configuration that user can modify

This INTRASITE_LINK_REGEX is a string. It should have the capturing
group name which is `what`.

This change was made after discussions with @ametaireau and
@justinmayer.

1. https://github.com/getpelican/pelican/pull/1067
1. https://github.com/getpelican/pelican/pull/1071

Updates getpelican/pelican#1061
Update settings documentation to explain INTRASITE_LINK_REGEX

I have added reference to "Linking to internal content" section because
without it, it is difficult to explain the usage.

I have also updated changelog.

Closes getpelican/pelican#1061