[pipes] update PipesContext variable name: context -> pipes

[yuhan]

Summary & Motivation

instead of context, we name PipesContext variables pipes. this PR also updates some docs copy to eliminate the mention of the "context" name.

pipes.report_x feels great.

but a few places where pipes feel odd:

• when it's accessing a property through "the context made available to pipes process", e.g. pipes.asset_keys, pipes.partition_key, pipes.log.info. the apis are the same as e.x. context.log.info in asset body but the variable naming convention make it feel different.
• pipes = PipesContext.get() imo context feels more natural in this line. but this might be minor?

How I Tested These Changes

[yuhan]

Current dependencies on/for this PR:

• master
  • PR [pipes] update PipesContext variable name: context -> pipes #17173 👈
    • PR [pipes] experiment: PipesContext -> Pipes #17176

This comment was auto-generated by Graphite.

[schrockn]

We could rename PipesContext to just Pipes

[github-actions]

Deploy preview for dagster-docs ready!

Preview available at https://dagster-docs-hgedf005r-elementl.vercel.app
https://10-12--pipes-update-PipesContext-variable-name-context---pipes.dagster.dagster-docs.io

Direct link to changed pages:

• https://dagster-docs-hgedf005r-elementl.vercel.app
  https://10-12--pipes-update-PipesContext-variable-name-context---pipes.dagster.dagster-docs.io/guides/dagster-pipes/subprocess/modify-external-code
• https://dagster-docs-hgedf005r-elementl.vercel.app
  https://10-12--pipes-update-PipesContext-variable-name-context---pipes.dagster.dagster-docs.io/guides/dagster-pipes/subprocess/reference

[yuhan]

closing as this variable rename is a no-go per slack convo