DOC Add section on ORM performance

[GuySartorelli]

This is just a couple of things for now - there are definitely more items that should be added here.

This section was mostly added because I needed a place to put the documentation for silverstripe/silverstripe-framework#10746 - but I bet there are a ton of additional bits and bobs we can document here as well (such as silverstripe/silverstripe-framework#10735 when it's merged)

I've created a separate card to look for other items that should be documented here.

Issue

• Allow skipping specific DataObject models from "check and repair" during dev/build silverstripe-framework#10495

[emteknetnz]

@GuySartorelli I probably merged this too soon, for 06_ORM.md could you open a new PR to mention that it'll disable the check and repair for the tables on the entire inheritance chain, both ancestors and descendants

I'm fairly sure that's accurate? I tested it with both

SilverStripe\ORM\Connect\DBSchemaManager:
  exclude_models_from_db_checks:
    - SilverStripe\CMS\Model\SiteTree

and

SilverStripe\ORM\Connect\DBSchemaManager:
  exclude_models_from_db_checks:
    - Page

using a breakpoint on a temporary variable

foreach ($ignoreDataObjects as $ignoreModel) {
    $_dcs = ClassInfo::dataClassesFor($ignoreModel); // <<< break point here

For both configurations it skip the check and repair for the models SiteTree, Page, ErrorPage, etc - implying that the "inheritance chain" goes in both directions

Let me know if I've misunderstood this

[GuySartorelli]

Yup that's correct - the unit test even explicitly ensures that's the case. I'll raise a new PR to add that context.