Clarify install docs

[connorjward]

Description

• 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

Fixes #4097 and #4098

Note that the docs link check is expected to fail because it references a link that doesn't yet exist.

TODOs

• Point ARM users to https://github.com/scientificcomputing/vtk-aarch64 to get ARM wheels for VTK (--extra-index-url?)

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake complex	8234 ran	6590 passed	1644 skipped	0 failed

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake default	8275 ran	7573 passed	702 skipped	0 failed

[JHopeCollins]

All good improvements. I just have a few suggestions for the PETSc installation details

[rckirby]

While clarifying install docs, perhaps we should add git+https to the suggested cloning commands for developer-mode install. Currently, it is http-only and one can't go over ssh/is prompted to supply GitHub credentials if one has followed the commands too slavishly.

[rckirby]

We need to make sure downstream packages installed in developer mode get installed via git

[connorjward]

We need to make sure downstream packages installed in developer mode get installed via git

@rckirby I'm not quite sure what you mean by this?

Addressed.

[rckirby]

But please delete the petsc/slepc version changes

[connorjward]

Forcing this through as it was previously approved and is holding up other PRs.