Add a Intro to the crosslinking page

[sureshhewabi]

Add a Intro to the crosslinking page:

PRIDE Crosslinking provides a centralised resource for accessing public cross-linking mass spectrometry (XL-MS) datasets within PRIDE. For datasets to be included in the resource, the crosslinking results need to be provided in the mzIdentML data standard (see detailed requirements below). PRIDE Crosslinking offers an infrastructure to improve the FAIRness of crosslinking data, including the web application xiVIEW for data visualisation, and cross-references to structural biology resources such as PDBe-KB, PDB-IHM and AlphaFoldDB.