min/max operators (#1992)

* min/max operators

* min, max operator headers

pkg/yqlib/doc/operators/headers/max.md

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+# Max
+
+Computes the maximum among an incoming sequence of scalar values.

pkg/yqlib/doc/operators/headers/min.md

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+# Min
+
+Computes the minimum among an incoming sequence of scalar values.

pkg/yqlib/doc/operators/max.md

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+
+## Maximum int
+Given a sample.yml file of:
+```yaml
+- 99
+- 16
+- 12
+- 6
+- 66
+```
+then
+```bash
+yq 'max' sample.yml
+```
+will output
+```yaml
+99
+```
+
+## Maximum string
+Given a sample.yml file of:
+```yaml
+- foo
+- bar
+- baz
+```
+then
+```bash
+yq 'max' sample.yml
+```
+will output
+```yaml
+foo
+```
+
+## Maximum of empty
+Given a sample.yml file of:
+```yaml
+[]
+```
+then
+```bash
+yq 'max' sample.yml
+```
+will output
+```yaml
+```
+

pkg/yqlib/doc/operators/min.md

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+
+## Minimum int
+Given a sample.yml file of:
+```yaml
+- 99
+- 16
+- 12
+- 6
+- 66
+```
+then
+```bash
+yq 'min' sample.yml
+```
+will output
+```yaml
+6
+```
+
+## Minimum string
+Given a sample.yml file of:
+```yaml
+- foo
+- bar
+- baz
+```
+then
+```bash
+yq 'min' sample.yml
+```
+will output
+```yaml
+bar
+```
+
+## Minimum of empty
+Given a sample.yml file of:
+```yaml
+[]
+```
+then
+```bash
+yq 'min' sample.yml
+```
+will output
+```yaml
+```
+