[FEATURE] Update SQL/PPL docs for 1.3

[joshuali925]

@keithhc2 We added a few new features in SQL/PPL and have updated documentation in our repo. The docs in this repo should also be updated. Are we the ones who should do this? And how were the other docs converted to markdown from rst? thanks

Documentation updates:

• add date_nanos data type: https://github.com/opensearch-project/sql/pull/360/files#diff-e4d5153d4f3ee0bebe5c5f00d573beae96e9eb0ee431da89c3cc41762ca3fcb2
• add parse command: https://github.com/opensearch-project/sql/pull/411/files#diff-d1e21c8ad5d6e5e687a31d1c69ec88be957855eed4aef2b89cfe98659a55d635
• add cast function: https://github.com/opensearch-project/sql/pull/433/files#diff-2cd6bff3d6c2cac7daebd7d067ebba2c545feb4c8332201f6bf78cede14b0b52
• Add parse docs to PPL commands index sql#486
• Add limitation section in PPL docs sql#456

[Naarcha-AWS]

@joshuali925: Hi Josh! Nate here, the new writer for OpenSearch. Let's address your questions individually:

1. Yes, feel free to add to add any changes you want to see in the docs via a Pull Request, and then tag me for review. For this particular release, I'll add your changes.
2. For markdown to rst conversion, I like to use a tool called Pandoc: https://pandoc.org/ . It's not perfect, but provides a decent enough output that fixing any formatting issues shouldn't take too long.

[Naarcha-AWS]

Closing in advance of 1.3.