Ensure that all references to creating and reviewing PRs mention blurb and NEWS entries

Other than for the most trivial sorts of changes (e.g. obvious spelling errors), nearly all changes to the cpython repo should include a NEWS entry; that's our primary mechanism to communicate changes with downstream users. We should be encouraging everyone, including non core-developers to submit PRs with NEWS entries. Among other benefits, having a NEWS entry makes reviews of the PR easier and helps to focus everyone include the submitter on what problem the PR is trying to address. And the tool we have to create NEWS entry is blurb; we want to encourage everyone creating or modifying a PR to use it. It's easier to delete the occasional unnecessary NEWS item than it is to create one when committing. To that end, suggest:

1. Create a section in the Git Bootcamp and Cheat Sheet Accepting and Merging section to describe the details of using blurb with a fallback to how to manually create a MiscNEWS.d entry: basically, adapt and move the detailed info from the Accepting Pull Requests section. We want to be careful to encourage blurb usage and not encourage manually creation as the details could change in the future.

2. This section should also include some guidelines on what a good NEWS entry looks like, or, if not here, one place in the devguide with references to it.

3. Review the whole document and add mentions of NEWS entries and the blurb command everywhere creating or reviewing or committing a PR is discussed and include a link to the detailed section created above. In particular, the Lifecycle of a Pull Request section needs to be edited to include blurb.

Lee Git Bootcamp y Cheat Sheet, la sección Accepting Pull Requests y la sección Lifecycle of a Pull Request en el devguide. Consolida el texto breve existente y las indicaciones de MiscNEWS.d en la sección Git Bootcamp, añade indicaciones sobre buenas entradas de NEWS y enlázala en todos los lugares donde se hable de crear, revisar o hacer commits de PRs.