Fix typo and markdown lint rules 022 and 036 (#934)

See:

- https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md022---headings-should-be-surrounded-by-blank-lines
- https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md036---emphasis-used-instead-of-a-heading

Author: Guts

README.md

@@ -25,7 +25,8 @@ details and [documentation](https://isso-comments.de/docs/).
 
 ## Getting started
 
-**Requirements**
+### Requirements
+
 - Python 3.7+ (+ devel headers)
 - SQLite 3.3.8 or later
 - a working C compiler
@@ -49,20 +50,24 @@ A Docker image with the latest stable release is provided at
 [Using Docker](https://isso-comments.de/docs/reference/installation/#using-docker).
 
 ## Contributing
+
 - Pull requests are very much welcome! These might be
   [good first issues](https://github.com/posativ/isso/labels/good-first-issue)
 - See [Ways to Contribute](https://isso-comments.de/docs/contributing/)
 - [Translate](https://isso-comments.de/docs/contributing/#translations)
 
 ### Development
+
 <!-- TODO also mention "Development & Testing" section once new docs uploaded -->
 Refer to the docs for
 [Installing from Source](https://isso-comments.de/docs/reference/installation/#install-from-source).
 
 ### Help
+
 - Join `#isso` via [Matrix](https://matrix.to/#/#isso:libera.chat) or via IRC on
   [Libera.Chat](https://libera.chat/)
 - Ask a question on [GitHub Discussions](https://github.com/posativ/isso/discussions).
 
 ## License
+
 MIT, see [LICENSE](LICENSE).