Merge branch 'feature/doc_todo_pages' into 'master'

Replace dead links by WIP doc page

See merge request !220

docs/reference/Adaptation.md

+# Adaptation {: #group-adaptation }
+
+This doc page is under construction.
+
+See the reference page on initfile parameters for [`Markers`](../Initfile#markers), [`Grids`](../Initfile#grids) and [`Adaptivity`](../Initfile#adaptivity).

docs/reference/Constraints.md

+# Constraints
+
+This doc page is under construction.
+
+See the reference page on the Problem class for information on using the function [`addDirichletBC`](../Problem/#function-problemstatadddirichletbc).

docs/reference/Filewriters.md

+# Filewriters
+
+This doc page is under construction.
+
+See the reference page on initfile parameters for [`Filewriters`](../Initfile#filewriters).

docs/reference/Solvers.md

+# Solvers
+
+This doc page is under construction.
+
+See the reference page on initfile parameters for [`Solvers`](../Initfile#solvers-and-preconditioners).

docs/tutorials/tutorials.md

@@ -23,23 +23,23 @@ problem and how to define your own operators.
 5. [Block Matrices and Vectors](block-mat-vec.md)
 Use block matrices and vectors instead of the default flat versions.
 
-6. [Boundary Conditions]():
+6. [Boundary Conditions](../reference/Constraints.md):
 Select parts of the grid boundary and assign boundary conditions or boundary
 operators. Learn how to specify the boundary parts and how to set different
 types of boundary conditions, like Dirichlet or periodic conditions, or
 Neumann/Robin-type boundary integrals.
 
-7. [Linear Solvers]():
+7. [Linear Solvers](../reference/Solvers.md):
 Control the solution of the assembled linear system, which linear solver to use
 and which preconditioner. How to control solver parameters and how to add an
 own linear solver.
 
-8. [Adaptivity and Grid Refinement]():
+8. [Adaptivity and Grid Refinement](../reference/Adaptation.md):
 Grid adaptivity needs control of marking and refinement strategies. Learn how to
 define and add a marker to the problem, learn about interpolation during grid
 adaptivity and how to load balance your grid.
 
-9. [FileWriters]():
+9. [FileWriters](../reference/Filewriters.md):
 Output and backup/restore of data is an important topic for simulations and
 for postprocessing. Different Writers are introduced and the configuration and control
 of these writers discussed.