Add link to detailed Net::HTTP documentation in README Usage section (#276)

## Problem
When looking at the [README](https://github.com/ruby/net-http?tab=readme-ov-file) on GitHub, it's unclear where to find more detailed information.

The [README](https://github.com/ruby/net-http?tab=readme-ov-file#usage) for the `Net::HTTP` gem includes the following sections:

- Overview
- Installation
- Usage
  - GET
  - GET by URI
  - GET with Dynamic Parameters
  - POST
  - POST with Multiple Values
- Development
- Contributing

The file [net-http/doc/net-http/examples.rdoc](https://github.com/ruby/net-http/blob/master/doc/net-http/examples.rdoc) also contains some simple usage examples.

However,  it is not clear where to look for following information:

- Details of the `get` or `post_form` methods used in the sample code:
  - Supported argument types
  - Return values
- Other ways to implement GET or POST requests
- Links to documentation covering the above

---

## Objective
To allow users of the `net-http` gem to easily find detailed information and sample code for implementing basic [HTTP requests](https://developer.mozilla.org/ja/docs/Web/HTTP/Reference/Methods) using `net-http`.

---

## Proposal
Add a link at the bottom of the README's [Usage](https://github.com/ruby/net-http?tab=readme-ov-file#development) section pointing to documentation with more detailed information.

---

The link is added only, without duplicating examples from rdoc, to avoid divergence between the README and official documentation.

Author: tkf0029

README.md

@@ -81,6 +81,8 @@ res = Net::HTTP.post_form(uri, 'q' => ['ruby', 'perl'], 'max' => '50')
 puts res.body
 ```
 
+See the [documentation](https://docs.ruby-lang.org/en/master/Net/HTTP.html) for more details.
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake test` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.