Deprecation guideline

It would be useful for the docs to have our deprecation policy spelled out.

Also having an official "deprecation text" would be useful so we can be more consistent across Ocean. I was speaking with @JoelPasvolsky and we recommend something like
```python
warnings.warn(
" is deprecated since X.Y.Z and will be removed in (X+2).0.0. "
"Use ."
DeprecationWarning, stacklevel=...
)
```
in the code and
```rst
.. deprecated:: X.Y.Z

will be removed in (X+2).0.0. Use .
```
in the docstring. `.. deprecated` prepends `Deprecated since version X.Y.Z:` to the notice.

Start by reviewing the existing documentation and deprecation uses around Python's warnings.warn entry point and the reStructuredText .. deprecated directive. Define the project-wide policy and consistent warning and docstring wording, then document both templates and their removal-version conventions.

Written by the indexing model from the issue text.