Update ADRSolver user guide

Issue/feature addressed

This MR update the ADRsolver user-guide.

• Update example for Helmholtz solver
• Update example for UnsteadyAdvection solver
• Add example for UnsteadyDiffusion solver
• Add example for UnsteadyReactionDiffusion solver
• Tidy-up UnsteadyReactionDiffusion.cpp (add explicit time-stepping provision (see MR !1731 (merged))

To be completed (separate MR):

• Add example for Projection solver
• Update example for UnsteadyAdvection solver (GJPStabilization, Dealiasing)
• Add example for UnsteadyInviscidBurgers solver (require !1718 (merged) to be merged)
• Add example for UnsteadyViscousBurgers solver (require !1718 (merged) to be merged)
• Update example for UnsteadyAdvectionDiffusion solver (SVV, Mixed CG/sub-stepping)
• Add example for SteadyAdvectionDiffusion solver
• Add example for SteadyAdvectionDiffusionReaction solver

Proposed solution

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

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Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).