Update lookup documentation (#1675 LOG4J2-3504)

[yakovsh]

• This pull requests updates the Property Substitution section of the configurations document page to list all currently valid lookups in Log4J.
• Adds information about the resource bundle lookup to the lookups page
• It also removes the information about the base64 lookup as per LOG4J2-3504
• Fixes link to the Log4J docker page

[yakovsh]

I signed the ICLA and sent it in, my Apache is id "yakovsh"

[vy]

Looks great @yakovsh! Thanks so much! I am merging this and I will create a changelog entry too.