Fix FISTA and ISTA maths in docs

[lauramurgatroyd]

Description

Closes #2013

Changes

Updated FISTA docs:

Updated ISTA docs:

Related issues/links

Closes #2013

Checklist

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have updated the relevant documentation
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties

[MargaretDuff]

Thanks Laura!

On lines 40-41 please can we replace "where :math:f is differentiable, :math:g has a simple proximal operator and :math:\alpha^k
is the :code:step_size per iteration."

with

"where :math:f is differentiable, :math:g has a simple proximal operator and :math:\alpha
is the :code:step_size."

and do the same for FISTA, if required.

[lauramurgatroyd]

There's a load of methods we expsose which we shouldnt in the docs so I am going to remove them:

[MargaretDuff]

Hi @lauramurgatroyd - looking good now and removing the extra information in FISTA has really helped readability.

Being picky now, please could we rearrange the intro to FISTA/ISTA so it reads:

• The XXX algorithm is used to solve $$min f(x)+g(x)$$ where f is ... and g is...
• In each update, the algorithm completes the following step(s)...
• Remaining notes and definitions

I think this gets the important information over quicker: what the algorithm does and what is required from the functions.

Being very picky, we should also not start a sentence with a mathematical expression. Words like "Where", "For" etc. can be used to start a sentence

[lauramurgatroyd]

[MargaretDuff]

Thanks @lauramurgatroyd