Include 2.4.x (Seam) to 3.1.x (Weld) script migration guide

[yurem]

It's not enough to just follow upgrade guide to do CE upgrade. Most of the customers are using own custom authentication scripts. We need to write initial version of this script migration document. After that we can add links to it from Script documentation and from Upgrade guide.

In order to write base version of this page we can use these 2 posts:

• https://support.gluu.org/authentication/6629/credetials-gone/#at44637
• https://support.gluu.org/authentication/6629/credetials-gone/#at44646
• https://support.gluu.org/authentication/6629/credetials-gone/#at44722

After getting more migration questions we will add new points to this page.