Add PostgreSQL-Compatibility, SQL Feature Coverage, PG Extensions

[ymahajan]

1. SQL Feature Support - To understand which standard SQL features we support
2. PostgreSQL compatibility - Since YugabyteDB is a distributed database, supporting all PostgreSQL features easily in a distributed system is not always feasible. This page documents the known list of differences between PostgreSQL and YugabyteDB.
3. PostgreSQL extensions- PostgreSQL provides a way to extend the functionality of a database by bundling SQL objects into a package and using them as a unit. Adding a page that describes the PostgreSQL extensions supported by YugabyteDB. These extensions can either be pre-bundled or may require installation.

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[aishwarya24]

Left some comments.

[aishwarya24]

This looks to me like the features that are under Going beyond SQL section: https://docs.yugabyte.com/latest/explore/ysql-language-features/. If that's the case, we may just need to link it to that page and not duplicate it to two pages.

[ymahajan]

I will remove this sub-section altogether, this doesn't have any concrete content yet.

[tichimura]

@ymahajan @polarweasel could you please update the link list of features currently supported by YSQL in https://github.com/yugabyte/yugabyte-db#core-features along with this changes?