Added JSON conversion tests and doc generation

pkg/yqlib/doc/usage/convert.md

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+# JSON
+
+Encode and decode to and from JSON. Note that YAML is a _superset_ of JSON - so `yq` can read any json file without doing anything special.
+
+This means you don't need to 'convert' a JSON file to YAML - however if you want idiomatic YAML styling, then you can use the `-P/--prettyPrint` flag, see examples below.
+
+## Parse json: simple
+JSON is a subset of yaml, so all you need to do is prettify the output
+
+Given a sample.json file of:
+```json
+{"cat": "meow"}
+```
+then
+```bash
+yq e -P '.' sample.json
+```
+will output
+```yaml
+cat: meow
+```
+
+## Parse json: complex
+JSON is a subset of yaml, so all you need to do is prettify the output
+
+Given a sample.json file of:
+```json
+{"a":"Easy! as one two three","b":{"c":2,"d":[3,4]}}
+```
+then
+```bash
+yq e -P '.' sample.json
+```
+will output
+```yaml
+a: Easy! as one two three
+b:
+  c: 2
+  d:
+    - 3
+    - 4
+```
+
+## Encode json: simple
+Given a sample.yml file of:
+```yaml
+cat: meow
+```
+then
+```bash
+yq e -o=json '.' sample.yml
+```
+will output
+```json
+{
+  "cat": "meow"
+}
+```
+
+## Encode json: simple - in one line
+Given a sample.yml file of:
+```yaml
+cat: meow # this is a comment, and it will be dropped.
+```
+then
+```bash
+yq e -o=json -I=0 '.' sample.yml
+```
+will output
+```json
+{"cat":"meow"}
+```
+
+## Encode json: comments
+Given a sample.yml file of:
+```yaml
+cat: meow # this is a comment, and it will be dropped.
+```
+then
+```bash
+yq e -o=json '.' sample.yml
+```
+will output
+```json
+{
+  "cat": "meow"
+}
+```
+
+## Encode json: anchors
+Anchors are dereferenced
+
+Given a sample.yml file of:
+```yaml
+cat: &ref meow
+anotherCat: *ref
+```
+then
+```bash
+yq e -o=json '.' sample.yml
+```
+will output
+```json
+{
+  "cat": "meow",
+  "anotherCat": "meow"
+}
+```
+
+## Encode json: multiple results
+Each matching node is converted into a json doc. This is best used with 0 indent (json document per line)
+
+Given a sample.yml file of:
+```yaml
+things: [{stuff: cool}, {whatever: cat}]
+```
+then
+```bash
+yq e -o=json -I=0 '.things[]' sample.yml
+```
+will output
+```json
+{"stuff":"cool"}
+{"whatever":"cat"}
+```
+

pkg/yqlib/doc/usage/headers/convert.md

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+# JSON
+
+Encode and decode to and from JSON. Note that YAML is a _superset_ of JSON - so `yq` can read any json file without doing anything special.
+
+This means you don't need to 'convert' a JSON file to YAML - however if you want idiomatic YAML styling, then you can use the `-P/--prettyPrint` flag, see examples below.