docs: streamline tutotiral

[eyurtsev]

Some changes:

1. Remove information about different PGEngine vectorization
2. Remove add_texts API. Add_documents is the best way to do this and we don't need to show two ways of doing the same stuff.
3. Replace "best practices" with concrete reasons why

Additional changes:

1. Would be nice to capture the exception more narrowly for schema creation.
2. More restructing would be good to merge creation of the table w/ metadata columns with the initial table creation.

from langchain_postgres import Column
from langchain_postgres import PGVectorStore


TABLE_NAME = "my_vectorstore_2"

await pg_engine.ainit_vectorstore_table(
    table_name=TABLE_NAME,
    vector_size=VECTOR_SIZE,
    metadata_columns=[
        Column("location", "TEXT"), 
        Column("topic", "TEXT"), 
    ],
)

[eyurtsev]

closed in favor of #193