pivot operator (#1993)

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

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+# Pivot
+
+Emulates the `PIVOT` function supported by several popular RDBMS systems.

pkg/yqlib/doc/operators/pivot.md

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+# Pivot
+
+Emulates the `PIVOT` function supported by several popular RDBMS systems.
+
+## Pivot a sequence of sequences
+Given a sample.yml file of:
+```yaml
+- - foo
+  - bar
+  - baz
+- - sis
+  - boom
+  - bah
+```
+then
+```bash
+yq 'pivot' sample.yml
+```
+will output
+```yaml
+- - foo
+  - sis
+- - bar
+  - boom
+- - baz
+  - bah
+```
+
+## Pivot sequence of heterogeneous sequences
+Missing values are "padded" to null.
+
+Given a sample.yml file of:
+```yaml
+- - foo
+  - bar
+  - baz
+- - sis
+  - boom
+  - bah
+  - blah
+```
+then
+```bash
+yq 'pivot' sample.yml
+```
+will output
+```yaml
+- - foo
+  - sis
+- - bar
+  - boom
+- - baz
+  - bah
+- -
+  - blah
+```
+
+## Pivot sequence of maps
+Given a sample.yml file of:
+```yaml
+- foo: a
+  bar: b
+  baz: c
+- foo: x
+  bar: y
+  baz: z
+```
+then
+```bash
+yq 'pivot' sample.yml
+```
+will output
+```yaml
+foo:
+  - a
+  - x
+bar:
+  - b
+  - y
+baz:
+  - c
+  - z
+```
+
+## Pivot sequence of heterogeneous maps
+Missing values are "padded" to null.
+
+Given a sample.yml file of:
+```yaml
+- foo: a
+  bar: b
+  baz: c
+- foo: x
+  bar: y
+  baz: z
+  what: ever
+```
+then
+```bash
+yq 'pivot' sample.yml
+```
+will output
+```yaml
+foo:
+  - a
+  - x
+bar:
+  - b
+  - y
+baz:
+  - c
+  - z
+what:
+  -
+  - ever
+```
+