README-MULTISITE: Fix syntax issues from markdownlint

This commit makes the following changes:

- Remove trailing whitespace;
- Use consistent header levels;
- Fix code blocks;
- Remove hard tabs;
- Fix ordered lists;
- Fix bare URLs;
- Use markdown list of sections.

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>
(cherry picked from commit 2c24442)