[DOCS] line length inconsistency

Is there an existing issue for this?

• I have searched the existing issues

This is a CLI Docs Problem, not another kind of Docs Problem.

• This is a CLI Docs Problem.

Description of Problem

Very minor! Two things I noticed while working on https://github.com/npm/cli/pull/5521, that other careful editors may be confused by (not know what the norms are):

1. Lines longer than 80 characters (outside of codeblocks).

I searched for:
^.{81}
(including *.md files, and excluding CHANGELOG.md,CHANGELOG-*.md files)

Seems like there are lots of exceptions*, should those be fixed, and should that norm (of wrapping in docs .md files) be documented somewhere? Or maybe it makes more sense to get rid of line limits in doc .md files, if it's not going to be enforced?

*: 131, if you also exclude CONTRIBUTING.md,DEPENDENCIES.md,README.md,SECURITY.md, but this count also includes code blocks.

2. Two spaces after periods.

I searched for:
\. {2}[^. ]: 471 results
and:
\. {1}[^. ]: 840 results
(including *.md files, and excluding CHANGELOG.md,CHANGELOG-*.md files)

So incredibly minor, probably not worth fixing, but thought I'd check -- when editing, what is preferred? One space is "winning", so I assumed that's the norm, and changed it in the paragraphs I rewrapped.

Potential Solution

1. Lines longer than 80 characters (outside of codeblocks).

Fix, and document norms in a discoverable place. CONTRIBUTING.md could have a section, or docs/README.md could be used for this purpose, or docs/CONTRIBUTING.md could be created and used.

2. Two spaces after periods.

These seems borderline too minor to fix or document, but I'm still tempted to fix some, just to tip the scales even more decisively, so it's more clear what the norm is. Assuming that's one space, if it's two, then there would be a bit more work to do to make that clear.

Start with CONTRIBUTING.md and docs/README.md, then reproduce the reported searches for Markdown files while noting the listed exclusions. Clarify the accepted line-length and spacing conventions, document the decision in the chosen contributor-facing location, and make any agreed documentation updates.