Fix broken links (Guidelines, Slack) (#32)

* Fix broken links to Guidelines page.

* Fix broken links to Slack workspace.

As mentioned in securego/gosec/issues/899, the heroku links are broken. This
link works for people who are already invited to the Slack workspace.

Someone with Slack admin permissions should probably generate an invite link, see:
https://slack.com/help/articles/201330256-Invite-new-members-to-your-workspace#share-an-invitation-link

Author: thinkmassive

website/core/Footer.js

@@ -34,7 +34,7 @@ class Footer extends React.Component {
           </a>
           <div>
             <h5>Docs</h5>
-            <a href={this.docUrl('guidelines.html', this.props.language)}>
+            <a href={this.docUrl('rules/rule-intro.html', this.props.language)}>
               Secure development guidelines
             </a>
             <a href='https://godoc.org/github.com/securego/gosec'>
@@ -49,7 +49,7 @@ class Footer extends React.Component {
               rel="noreferrer noopener">
               Stack Overflow
             </a>
-            <a href="http://securego.herokuapp.com/">Slack</a>
+            <a href="https://securego.slack.com/">Slack</a>
           </div>
           <div>
             <h5>More</h5>

website/pages/en/help.js

@@ -27,13 +27,13 @@ class Help extends React.Component {
     const supportLinks = [
       {
         content: `Learn more using the [guidelines on this site.](${docUrl(
-          'guidelines.html',
+          'rules/rule-intro.html',
           language
         )})`,
         title: 'Browse Docs',
       },
       {
-        content: 'Ask us questions on [Slack](http://securego.herokuapp.com/).',
+        content: 'Ask us questions on [Slack](https://securego.slack.com/).',
         title: 'Join the community',
       },
       {