Ploomber

* Simplifies serializer and unserializer creation with new decorators
* Adds guide on serializer/unserializer decorators to the documentation

* Clearer error message when failing to import function
* Better error message when `tasks` in YAML spec is not a list
* Fixes an issue that caused dag plot to fail when using `.svg`
* Fixes duplicated log entries when viewing a file in Jupyter

* Fixes cell injection when using the `--notebook-dir` during Jupyter initialization
* Reduces verbosity in Jupyter logs ([314](https://github.com/ploomber/ploomber/issues/314))
* Adds `tasks[*].params.resources_` to track changes in external files
* Minor bug fixes

* Lazy load for `serializer`, `unserialize`, DAG clients, Task clients, Product clients, and task hooks, which allows the Jupyter plugin to work even if the Jupyter process does not have the dependencies required to import such dotted paths
* CLI `--help` message shows if `ENTRY_POINT` environment variable is defined
* `ploomber scaffold` now takes a `-e/--entry-point` optional argument
* Fixes error that caused the `{{here}}` placeholder not to work if an `env.yaml` exists
* Adds `--empty` option to `ploomber scaffold` to create a `pipeline.yaml` with no tasks

- Allowing `pipeline.yaml` at project root if setup.py but `src/*/pipeline.yaml` is missing
- Fixes bug in `EnvDict.find` that caused the `{{here}}` placeholder to point to the `env.yaml` file instead of its parent
- `DAGSpec._find_relative` returns relative path to spec
- Fixes error that missed `env.yaml` loading when initializing DAGSpecPartial

- Changes the logic that determines project root: only considers `pipeline.yaml` and `setup.py` (instead of `environment.yml` or `requirements.txt`)
- Adds configuration and scaffold user guides
- Updates Jupyter user guide
- Deletes conda user guide
- Renames internal modules for consistency (this should not impact end-users)
- Fixes error that caused Files generated from TaskGroups in the spec API not to resolve to their absolute values
- Fixes error that caused metadata not to delete on when saving files in Jupyter if using a source in more than one task
- `DAGSpec` loads an `env.{name}.yaml` file when loading a `pipeline.{name}.yaml` if one exists
- `ploomber plot` saves to `pipeline.{name}.png`
- Override `env.yaml` to load using `PLOOMBER_ENV_FILENAME` environment variable
- `EnvDict` init no longer searches recursively, moved that logic to `EnvDict.find`. `with_env` decorator now uses the latter to prevent breaking the API
- `PostgresCopyFrom` compatible with `psycopg>=2.9`
- `jupyter_hot_reload=True` by default
- `PythonCallableSource` finds the location of a dotted path without importing any of the submodules
- Jupyter integration lazily loads DAGs (no need to import callable tasks)
- CLI no longer showing `env.yaml` parameters when initializing from directory or pattern