[BUGFIX] Streamline README.md and INSTALL.md with current links

In the past year a lot of links have changed, especially in documentation
which is now reflected in the README.md and INSTALL.md files.

Resolves: #88956
Releases: master, 9.5
Change-Id: I754384bffb924340c59e18536edb41883ec279cb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63535


Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Benni Mack <benni@typo3.org>

CONTRIBUTING.md

@@ -13,9 +13,9 @@ If you like to contribute to our [documentation][5], use the "Edit me on GitHub"
  in the top right of each manual or reference or just make a pull request in any of the repositories
  that can be found in the TYPO3 [documentation team GitHub organisation][6].
 
-[1]: https://docs.typo3.org/typo3cms/ContributionWorkflowGuide/
+[1]: https://docs.typo3.org/core-contribution
 [2]: https://git.typo3.org/Packages/TYPO3.CMS.git
 [3]: https://review.typo3.org/#/q/project:Packages/TYPO3.CMS,n,z
 [4]: https://github.com/TYPO3/TYPO3.CMS
-[5]: https://docs.typo3.org/typo3cms/
+[5]: https://docs.typo3.org
 [6]: https://github.com/TYPO3-Documentation

INSTALL.md

@@ -8,13 +8,13 @@ Official Installation and Upgrade Guide
 ---------------------------------------
 
 You can find instructions regarding installation and upgrade at
-https://docs.typo3.org/typo3cms/InstallationGuide/
+https://docs.typo3.org/installation
 
 
 Server system requirements
 --------------------------
 
-Find the official system requirements at [get.typo3.org](https://get.typo3.org/version/9#system-requirements).
+Find the official system requirements in TYPO3's [documentation](https://docs.typo3.org/system-requirements).
 
 TYPO3 Security
 --------------

README.md

@@ -14,7 +14,7 @@ Getting Started
 TYPO3 requires a web server with PHP and a database (e.g. MySQL).
 Accessing the backend through a supported browser.
 
-Please see the [Installation Guide](https://docs.typo3.org/typo3cms/InstallationGuide/)
+Please see the [Installation Guide](https://docs.typo3.org/installation)
 in order to set up a basic TYPO3 installation on your web server.
 
 What is TYPO3?
@@ -62,8 +62,7 @@ get started:
 ### Get more information
 
 * https://typo3.org is the main project website. It provides up to date
-  official news, information about events, access to downloading the
-  products and extensions.
+  official news, information about events and the TYPO3 community.
 
 * https://docs.typo3.org/: TYPO3 is one of the most thoroughly
   documented OpenSource products around, with manuals covering basic
@@ -84,7 +83,7 @@ with each other and with the public. Several TYPO3 teams use Slack as a
 way to communicate internally and most channels are a welcome place for
 you to join and get yourself involved.
 
-* Register: https://forger.typo3.org/slack/
+* Register: https://my.typo3.org/about-mytypo3org/slack
 * Slack: https://typo3.slack.com/
 
 ### Exchange information, ask questions, get help
@@ -112,7 +111,7 @@ Contributing
 If you want to contribute to the TYPO3 source code, take a look at our
 Contributors Walkthrough and Review System:
 
-* https://docs.typo3.org/typo3cms/ContributionWorkflowGuide/
+* https://docs.typo3.org/core-contribution
 * https://review.typo3.org/
 
 > Please use the [TYPO3 Slack chat](#chat-with-us), if you need help in