feat(v2): Implement plugin creating feed for blog posts

[moozzyk]

Fixes: #1698

Motivation

Requested feature

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Test plan:

• added tests

Ran yarn build on website with the following config:

'@docusaurus/plugin-content-blog-feed',
{
  path: '../website-1.x/blog',
  feedOptions: {
    copyright: 'Copy',
    type: 'atom',
  },
},

which genereted the following feed:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
    <id>https://v2.docusaurus.io/blog</id>
    <title>Docusaurus Blog</title>
    <updated>2018-12-14T00:00:00.000Z</updated>
    <generator>https://github.com/jpmonette/feed</generator>
    <link rel="alternate" href="https://v2.docusaurus.io/blog"/>
    <subtitle>Docusaurus Blog</subtitle>
    <icon>img/docusaurus.ico</icon>
    <rights>Copy</rights>
    <entry>
        <title type="html"><![CDATA[Happy 1st Birthday Slash!]]></title>
        <id>Happy 1st Birthday Slash!</id>
        <link href="https://v2.docusaurus.io/blog/2018/12/14/Happy-First-Birthday-Slash"/>
        <updated>2018-12-14T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[![First Birthday Slash](/img/docusaurus-slash-first-birthday.svg)]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[Towards Docusaurus 2]]></title>
        <id>Towards Docusaurus 2</id>
        <link href="https://v2.docusaurus.io/blog/2018/09/11/Towards-Docusaurus-2"/>
        <updated>2018-09-11T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[Docusaurus was [officially announced](https://docusaurus.io/blog/2017/12/14/introducing-docusaurus) over nine months ago as a way to easily build open source documentation websites. Since then, it has amassed over 8,600 GitHub Stars, and is used by many popular open source projects such as [React Native](https://facebook.github.io/react-native/), [Babel](https://babeljs.io/), [Jest](https://jestjs.io/), [Reason](https://reasonml.github.io/) and [Prettier](https://prettier.io/).]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[How I Converted Profilo to Docusaurus in Under 2 Hours]]></title>
        <id>How I Converted Profilo to Docusaurus in Under 2 Hours</id>
        <link href="https://v2.docusaurus.io/blog/2018/04/30/How-I-Converted-Profilo-To-Docusaurus"/>
        <updated>2018-04-30T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[> _“Joel and I were discussing having a website and how it would have been great to launch with it. So I challenged myself to add Docusaurus support. It took just over an hour and a half. I'm going to send you a PR with the addition so you can take a look and see if you like it. Your workflow for adding docs wouldn't be much different from editing those markdown files.”_]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[Introducing Docusaurus]]></title>
        <id>Introducing Docusaurus</id>
        <link href="https://v2.docusaurus.io/blog/2017/12/14/introducing-docusaurus"/>
        <updated>2017-12-14T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[![Introducing Slash](/img/slash-introducing.svg)]]></summary>
    </entry>
</feed>

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[docusaurus-bot]

Deploy preview for docusaurus-2 ready!

Built with commit 28ba79c

https://deploy-preview-1916--docusaurus-2.netlify.com

[docusaurus-bot]

Deploy preview for docusaurus-preview ready!

Built with commit 28ba79c

https://deploy-preview-1916--docusaurus-preview.netlify.com

[endiliey]

On a first impression, it looks good but I think this should be inside the docusaurus-plugin-content-blog. So the blog plugin itself also create feeds for you.
I think that will remove a lot of duplicates code, they can toggle whether to generate the feed/not through option.

[moozzyk]

OK. I was curious about that. I actually have a prototype where this logic is part of the blog plugin. I did not like the duplicated code as well and also the fact that if this code goes out of sync the feed may no longer be correct. I found this comment in the blog plugin which made me think there is an effort to separate common blog functionality planned which all the plugins would reuse.
I think moving this logic should not be that heard given it is implemented as a post-build step.

[endiliey]

Thank you, looking forward to it. You are awesome 😉

[moozzyk]

🆙📅 - integrated feed with the blog plugin

[endiliey]

I tweaked it a little bit so that i can generate both rss and atom feed with feedType = 'all'. I also added minimal docs

Overall it looks good to me.

@yangshun look ok to u ?

[endiliey]

I added some documentation, it will be nice if you were to update it further @moozzyk