Document how to handle type conflicts on write with v3

With InfluxDB v3 (and all previous version of InfluxDB), if you try to write data to a column using a different type that what's already in the column, it errs. With v3, because InfluxDB v3's SQL and InfluxQL implementations are read-only, you can't write queried/manipulated data back into the database at query time so you can't just cast the column to a new type and write it back in. You have to do one of the following:

- Query your data out of InfluxDB into an external tool/runtime, then write it back in.
- Store the data of a different type in a new column.

Start by locating the InfluxDB v3 write and data-type documentation in this repository, then verify the stated behavior for type conflicts and the read-only SQL and InfluxQL implementations. Done means documenting the two supported alternatives: exporting data to an external tool or storing the different type in a new column.

Written by the indexing model from the issue text.