Tidy a few more links

Fixing issues spotted after rebasing.

* Retitle "explainer" to "explanation" for consistency with other pages
* Move a Data Builder page that was at the top-level, and fix relative
  links in that document

docs/_redirects

@@ -74,8 +74,8 @@
 /ehrql-intro/                            /data-builder/ehrql/intro/
 /ehrql-reference                         /data-builder/ehrql/reference/
 /ehrql-reference/                        /data-builder/ehrql/reference/
-/ehrql-set-population-explainer          /data-builder/ehrql/set-population-explainer/
-/ehrql-set-population-explainer/         /data-builder/ehrql/set-population-explainer/
+/ehrql-set-population-explainer          /data-builder/ehrql/set-population-explanation/
+/ehrql-set-population-explainer/         /data-builder/ehrql/set-population-explanation/
 /ehrql-new-tutorial-1a                   /data-builder/ehrql/tutorial/1a/
 /ehrql-new-tutorial-1a/                  /data-builder/ehrql/tutorial/1a/
 /ehrql-new-tutorial-1b                   /data-builder/ehrql/tutorial/1b/

docs/data-builder/ehrql/tutorial/3a.md

@@ -69,7 +69,7 @@ This introduces a distinction between *patient-level* and *event-level* tables:
 * `patient` is a patient-level table where one row represents one patient
 * `patient_address` is event-level where each row is an event and a patient can have many events.
    In this case, someone might move house and have a new address.
-   See the [explainer](data-builder-patient-event-tables) for more information.
+   See the [explainer](../../patient-event-tables-explanation.md) for more information.
 
 ### Address by date
 

docs/data-builder-patient-event-tables-explanation.md → docs/data-builder/patient-event-tables-explanation.md

@@ -89,7 +89,7 @@ in a way that ehrQL does not support.
 
 The full details are out of scope for this tutorial.
 When you have completed the tutorial,
-you may wish to review the [more detailed explanation](ehrql-combining-series-and-frames.md).
+you may wish to review the [more detailed explanation](ehrql/combining-series-and-frames.md).
 
 !!! todo
     This is currently on its own page.
@@ -109,7 +109,7 @@ you can perform simple mathematical operations on column (series) values
 including combining with other column values for a patient,
 or combining with some other value specified or computed in the dataset definition.
 
-See ["Combining series"](ehrql-reference.md/#5-combining-series) in the ehrQL reference.
+See ["Combining series"](ehrql/reference.md/#5-combining-series) in the ehrQL reference.
 
 !!! todo
     Review this URL.