Add documentation for the GUI for consistency check (#551)

Author: priyanshu16095

en/.gitbook/assets/checkconsistency.png

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+# Check consistency
+
+JabRef can check the consistency of a library.
+
+This feature is available through **Quality → Check consistency**.
+
+## Background
+
+You’re finalizing your research paper for an upcoming conference, and the deadline is near. While reviewing your references, you notice inconsistencies - some citations are missing DOIs others are missing page numbers. Manually identifying these across dozens of entries is tedious and time-consuming.
+
+JabRef now makes this process effortless with its Bibliography Consistency Check feature. It automatically scans your references, identifies missing or inconsistent fields, and presents a structured report, allowing you to fix issues with just a few clicks.
+
+## How to use
+
+1. Open JabRef and go to the Quality menu.
+2. Click on "Check Consistency" (below "Check Integrity").
+3. JabRef will run the check on your entire library and show the results in a new window.
+4. The result window will display entries grouped by their entry type (e.g., articles, books).
+5. Each entry will be shown in a table with columns indicating whether required fields are present (x), optional fields are present (o), or if a field is missing (-).
+6. If any entry has missing fields, click on it to go directly to the entry in the entry editor.
+
+## Checking a .bib File for Consistency
+
+Let’s say we have a .bib file like this:
+
+```bibtex
+@Article{Corti_2009,
+  author       = {Corti, Roberto and Flammer, Andreas J. and Hollenberg, Norman K. and Lüscher, Thomas F.},
+  title        = {Cocoa and Cardiovascular Health},
+  journaltitle = {Circulation}, 
+  issn         = {1475-2662}, 
+  volume       = {119}
+}
+
+@Article{Cooper_2007,
+  author       = {Cooper, Karen A. and Donovan, Jennifer L. and Waterhouse, Andrew L. and Williamson, Gary},
+  title        = {Cocoa and health: a decade of research},
+  issn         = {1743-7075},
+  volume       = {99}
+}
+```
+
+Here, the second entry is missing the journal field, which is required for an article. Running the Check Consistency tool will highlight this issue as shown:
+
+![Consistency check results](<../.gitbook/assets/consistencycheck_results.png>)
+
+## Results Window
+
+![Check consistency dialog](<../.gitbook/assets/checkconsistency.png>)
+
+The result window is designed to present the consistency check results in an easily digestible format:
+
+- **Entry Type Headings**: The first column lists the name of the selected entry type.
+- **Choose Entry type**: Entry types (such as article, book, in-proceedings, etc.) will be listed in a dropdown menu.
+- **Columns**:
+  - **Column 2**: Citation key of the entry.
+  - **Other columns**: Represent the fields and their status (`x`, `o`, `?`, or `-`).
+- **Navigation**: Clicking on a line in the table will take you directly to the corresponding entry in the editor, making it easy to address inconsistencies.
+
+## Symbols in the Results
+
+The following symbols will be used to indicate the presence or absence of fields:
+
+- `x`: Required field is present.
+- `o`: Optional field is present.
+- `?`: Unknown field is present.
+- `-`: Field is absent.
+
+This simple system helps you quickly identify entries that need attention.