Include more information about optimising ORM performance

https://github.com/silverstripe/developer-docs/pull/201 adds a new section about ORM performance. It doesn't have much in it right now, but there's probably a lot that we can include in here.
Examples:

• using PartialMatch filters makes indexes effectively useless for that column in that query - so for things like searching for pages in the CMS site tree, swapping to use ExactMatch on title only and adding an index on title will make that query way faster
• dnadesign/silverstripe-elemental overrides how sitetree search works and adds a super un-optimised filterByCallback() to that query (though its debatable whether elemental docs should be part of the main docs)

We should seek feedback from the community about what they're doing to improve performance of the ORM specifically and include good examples in the ORM performance section (pssst if you're reading this and have any suggestions of what we could add, please add a comment below).

Start by reading pull request 201 and its new ORM performance section. Review the listed PartialMatch, ExactMatch, indexing, and filterByCallback examples, then seek community feedback for additional ORM performance practices. Done means the section contains useful, vetted examples and clearly addresses the agreed scope, including whether Elemental documentation belongs there.