docs: improve/elaborate main project description

[knocte]

By just describing as 'style checking tool', one could have thought
of FSharpLint as a formatting tool (sort of an alternative to fantomas)
but we cover way more than that. And to give a better idea of what
kind of issues can be detected, we put three different kinds of rules
here.

[Copilot]

Pull Request Overview

This PR enhances the project’s main description by reframing FSharpLint as a static analysis tool and illustrating the kinds of rules it enforces.

• Updated the overview in docs/content/index.md to emphasize static analysis and list rule categories.
• Mirrored the expanded description and bullets in README.md.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/content/index.md	Expanded description to “static analysis tool” and added rule types
README.md	Updated summary to match docs and introduced a rule categories list

Comments suppressed due to low confidence (4)

docs/content/index.md:9

• [nitpick] The sentence is split mid-clause across two lines, which may lead to unintended paragraph breaks or readability issues. Consider merging it into a single line to ensure the description remains coherent in rendered Markdown.

FSharpLint is a static analysis tool for F#. Like many other software linters, it analyzes your source code (via typed and untyped syntax trees), and then it points out locations where a set of rules on how 

docs/content/index.md:12

• [nitpick] The phrase "API/ABI members" could be confusing for readers. Consider clarifying or rephrasing to "public API and ABI members" or briefly explaining what "ABI" refers to in this context.

* Conventions: name your public or private API/ABI members to respect project or solution-wide conventions.

README.md:3

• [nitpick] This very long line may reduce readability in raw Markdown. Consider splitting it into shorter sentences or wrapping at logical breaks for easier maintenance.

FSharpLint is a static analysis tool for F#. Like many other software linters, it analyzes your source code (via typed and untyped syntax trees), and then it points out locations where a set of rules on how F# is to be styled have been broken. There are various kinds of rules:

README.md:5

• [nitpick] As above, clarifying "API/ABI members" or explaining "ABI" would help readers who may not be familiar with the term.

* Conventions: name your public or private API/ABI members to respect project or solution-wide conventions.

[knocte]

@xperiandri thanks for your improvements, included them and merged; is English your mother tongue? Mine isn't 😭