Convert MongoDB documentation from a top element of set to book

[haszi]

Convert all underlying elements as needed:

• <books> to <part>s or <chapter>s
• <article>s to <section>s

Move the Predefined Constants page from the Installing/Configuring section to its own

Change the following xml:ids:

• book.mongodb to mongodb.mongodb
• set.mongodb to book.mongodb
• book.bson to mongodb.bson

Please note that these changes only work when the accompanying doc-base changes are merged too (https://github.com/php/doc-base/pull/138).

Note: Please note that because of the difference between root chunks and container chunks in PhD (ie. a <book> and a <part>), the four pages linked to from the main MongoDB page (MongoDB, BSON, Monitoring and Exceptions) are only going to list their underlying classes without listing each method of each class. E.g. the MongoDB Extension Classes page will only list the 19 classes belonging to that group of classes instead of listing the 19 classes plus all their 144 methods. As before, the methods are still listed on the pages of their respective classes.