Improvements for database setup docs

[mik-laj]

• First of all, the previous version had a title that did not describe its content well. I corrected it which will make it easier to find this page.
• I have added sections that describe the requirements for the supported database versions. Previously, there was only a link to HA requirements, but no description for the basic configuration.
• I have moved the sections on database URI to the top of this document as this knowledge is needed in the following sections.
• I have created separate sections for MySQL and PostgreSQL because users who use MySQL are not interested in the information for PostgresSQL and vice versa. This also allowed us to delete various blocks of notes that were difficult to format and read.

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^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[kaxil]

Suggested change

	Set up a MySQL
	Set up MySQL

or

Suggested change

	Set up a MySQL
	Setting up a MySQL Database

[kaxil]

Suggested change

	Set up a PostgreSQL
	-------------------
	Set up a PostgreSQL Database
	-----------------------------

or

Suggested change

	Set up a PostgreSQL
	-------------------
	Set up PostgreSQL
	-----------------------------

[kaxil]

Suggested change

You need to create a database and a user that Airflow will use to access this database.

already mentioned before the code-block

[kaxil]

Minor changes suggested or else LGTM

[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest master or amend the last commit of the PR, and push it with --force-with-lease.

[mik-laj]

@kaxil Updated. See: 1662aa0