Clarify install docs (#4121)

* Add section about linux ARM VTK
* Removes some references to firedrake-install
* Makes pip cache remove petsc4py a main instruction so it isn't missed
* Moves complex bits about customisation to the bottom of the install instructions
* Included explicit information about installing SLEPc and other optional dependencies

Author: connorjward

.github/workflows/build.yml

@@ -72,8 +72,11 @@ jobs:
             xargs -L1 ./configure --with-make-np=8 --download-slepc
           make PETSC_DIR=/__w/firedrake/firedrake/petsc PETSC_ARCH=arch-firedrake-${{ matrix.arch }}
           make check
-          echo "PETSC_DIR=/__w/firedrake/firedrake/petsc" >> "$GITHUB_ENV"
-          echo "PETSC_ARCH=arch-firedrake-${{ matrix.arch }}" >> "$GITHUB_ENV"
+          {
+            echo "PETSC_DIR=/__w/firedrake/firedrake/petsc"
+            echo "PETSC_ARCH=arch-firedrake-${{ matrix.arch }}"
+            echo "SLEPC_DIR=/__w/firedrake/firedrake/petsc/arch-firedrake-${{ matrix.arch }}"
+          } >> "$GITHUB_ENV"
 
       - name: Install Firedrake
         id: install

demos/eigenvalues_QG_basinmodes/qgbasinmodes.py.rst

@@ -18,10 +18,8 @@ dissipative forces. Unlike the other demo that integrated the equations forward
 in time, in this problem it is necessary to compute the eigenvalues and
 eigenfunctions for a particular differential operator.
 
-This demo requires SLEPc and slepc4py to be installed.  This is most easily
-achieved by providing the optional `--slepc` flag to either `firedrake-install`
-(for a new installation), or `firedrake-update` (to add SLEPc to an existing
-installation).
+This demo requires SLEPc and slepc4py to be installed. For instructions on how
+to install them please follow `these instructions <https://www.firedrakeproject.org/install#slepc>`_.
 
 
 Governing PDE

demos/netgen/netgen_mesh.py.rst

@@ -11,7 +11,7 @@ Finally we will show how to use mesh refinement features included in Netgen to c
 
 Installing Netgen
 ------------------
-This demo requires the NGSolve/Netgen suite to be installed. This is most easily achieved by providing the optional `--netgen` flag to either `firedrake-install` (for a new installation), or `firedrake-update` (to add the NGSolve/Netgen suite to an existing installation). ::
+This demo requires the NGSolve/Netgen suite to be installed. This can be done by running ``pip install ngsPETSc``. ::
 
    from firedrake import *
    try:

docs/source/checkpointing.rst

@@ -617,9 +617,9 @@ Firedrake uses the PETSc_ HDF5 Viewer_ object to write and read state.
 As such, writing data is collective across processes.  h5py_ is used
 for attribute manipulation.  To this end, h5py_ *must* be linked
 against the same version of the HDF5 library that PETSc was built
-with.  The ``firedrake-install`` script automates this, however, if
-you build PETSc manually, you will need to ensure that h5py_ is linked
-correctly following the instructions for custom installation here_.
+with. If you have a custom installation of Firedrake then it may be
+necessary to manually ensure that h5py is linked correctly.
+Instructions for how to do this can be found here_.
 
 .. warning::