Add documentation for new MG

refs #31851

Author: GiudGiud

framework/doc/content/source/meshgenerators/PolyLineFollowingNodeSetMeshGenerator.md

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+# PolyLineMeshFollowingNodeSetGenerator
+
+!syntax description /Mesh/PolyLineMeshFollowingNodeSetGenerator
+
+Using the `PolyLineMeshFollowingNodeSetGenerator` object from within the
+[Mesh](/Mesh/index.md) block of the input file will construct an open
+or closed (looped) one-dimensional manifold of Edge elements.
+
+The polyline is generated iteratively from the starting point with the starting direction:
+The following heuristics are implemented for following the nodeset:
+
+- Sphere-based nodeset centroid search
+    - the current point is moved by [!param](/Mesh/PolyLineMeshFollowingNodeSetGenerator/dx) towards the current direction
+    - the local centroid of all nodes in the target nodeset and closer than [!param](/Mesh/PolyLineMeshFollowingNodeSetGenerator/search_radius) to the current point is found
+    - the current direction is updated to be from the current point to the centroid
+    - the displacement of the current point is annulled, and replaced by a displacement of magnitude [!param](/Mesh/PolyLineMeshFollowingNodeSetGenerator/dx) towards the centroid
+- Ignoring nodes behind
+    - only nodes from the nodeset that are also ahead of the current point with the current direction are considered
+
+!alert note
+Because of the use of heuristics rather than a global solve, the 1D polyline solution is highly dependent on the parameters chosen.
+The user should tune these parameters to fit their needs. Notably, the number of edges can be reduced to prevent the polyline from
+backtracking after having reached the end of the nodeset.
+
+!syntax parameters /Mesh/PolyLineMeshFollowingNodeSetGenerator
+
+!syntax inputs /Mesh/PolyLineMeshFollowingNodeSetGenerator