Add docs outlining changes we made to `PostgresRow/column(name:)`

[fabianfett]

Added a doc that describes how to best migrate to the newer and better PostgresRow cell access API. While at it also opened up some convenience API's that make dealing with it much nicer.

[codecov-commenter]

Codecov Report

Merging #312 (6576e77) into main (a12d09f) will decrease coverage by 0.01%. The diff coverage is 40.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #312 +/- ## ========================================== - Coverage 43.53% 43.52% -0.02% ========================================== Files 115 115 Lines 9760 9763 +3 ========================================== Hits 4249 4249 - Misses 5511 5514 +3 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `43.52% <40.00%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/vapor/postgres-nio/pull/312?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [Sources/PostgresNIO/New/PostgresCell.swift](https://codecov.io/gh/vapor/postgres-nio/pull/312/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Qb3N0Z3Jlc05JTy9OZXcvUG9zdGdyZXNDZWxsLnN3aWZ0) | `88.23% <25.00%> (-8.54%)` | :arrow_down: | | [Sources/PostgresNIO/Data/PostgresRow.swift](https://codecov.io/gh/vapor/postgres-nio/pull/312/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Qb3N0Z3Jlc05JTy9EYXRhL1Bvc3RncmVzUm93LnN3aWZ0) | `49.24% <100.00%> (ø)` | |