Remove mention of the Knowledge Base, use direct links to new docs site

The following mentions of "Knowledge base" on should be changed to "Documentation", as follows, with suggested improvements to the wording:

1) "You can read more about the content of this preview in the Knowledge Base."
->
"Read more about the content of this preview in the Documentation." (see also link fix at MDBF-1080)

2) "The Reporting Bugs page on the Knowledge Base has details on how to report a bug."
->
"The Reporting Bugs page in the MariaDB Documentation describes how to report a bug."

3) The link for "https://mariadb.com/kb/en/mysql-test-run/" should go to "https://mariadb.com/docs/server/clients-and-utilities/testing-tools/mariadb-test"

4) The link for "https://mariadb.com/kb/en/debugging-mariadb-with-mysql-test-run/" should go to "https://mariadb.com/kb/en/debugging-mariadb-with-mysql-test-run/"

5) Please change the banner body:
Replace "It contains new features that we plan to include in the next MariaDB Server release. Try them out and please let us know of your experience. We aim to incorporate all the feedback we receive, which we can only do now, before the release, for compatibility reason."

With "This preview includes new features which can be tested before the official release. However, some features may not be included in the final <release_series> series if further testing detects problems."

6) -When downloading 12.1.0, the site redirects to https://mariadb.com/kb/en/postdownload/mariadb-server-12-1-0, which doesn't redirect (and doesn't have a matching page on the new docs site). I suggest removing this redirect until/if this page is created. Redirect pages now sit in the format: https://mariadb.com/docs/platform/post-download/mariadb-server-11.8.2- Fixed. Now, it's just the Connectors. They currently go to https://mariadb.com/docs/release-notes/connectors/, so at least no 404s. Sample end locations are:

• https://mariadb.com/docs/platform/post-download/mariadb-connector-c-2.2.3
• https://mariadb.com/docs/platform/post-download/mariadb-connector-j-1.6.2
• https://mariadb.com/docs/platform/post-download/mariadb-connector-nodejs-3.1.0
• https://mariadb.com/docs/platform/post-download/mariadb-connector-python-1.1.4