Added File operators

pkg/yqlib/doc/File Operators.md

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+The file operator is used to filter based on filename. This is most often used with merge when needing to merge specific files together.
+
+```bash
+yq eval 'filename == "file1.yaml" * fileIndex == 0' file1.yaml file2.yaml
+```
+## Examples
+### Get filename
+Given a sample.yml file of:
+```yaml
+'': null
+```
+then
+```bash
+yq eval 'filename' sample.yml
+```
+will output
+```yaml
+sample.yaml
+```
+
+### Get file index
+Given a sample.yml file of:
+```yaml
+'': null
+```
+then
+```bash
+yq eval 'fileIndex' sample.yml
+```
+will output
+```yaml
+73
+```
+

pkg/yqlib/doc/headers/File Operators.md

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+The file operator is used to filter based on filename. This is most often used with merge when needing to merge specific files together.
+
+```bash
+yq eval 'filename == "file1.yaml" * fileIndex == 0' file1.yaml file2.yaml
+```