Remove outdated reaction forces section from Phase_Field_Fracture doc…

[CastillonMiguel]

…umentation and add Reactions documentation with corresponding module details.

This pull request reorganizes the documentation structure by moving the "Reaction forces" section from the Element module to a new Reactions module and updates the API reference index to reflect this change.

Documentation restructuring:

• docs/source/api/Element/Phase_Field_Fracture/main.rst: Removed the "Reaction forces" section from the Element module documentation.
• docs/source/api/Reactions/main.rst: Created a new Reactions module documentation and added the "Reaction forces" section under it.

API reference update:

• docs/source/api/index.rst: Updated the API reference index to include the new Reactions/main entry.