GDScript: Update match patterns and literal docs

[dalexeev]

1. Fix nested lists:

2. Fix constant and variable patterns, they are actually literal and expression patterns.

3. Use level 5 headers for match subsections, it looks better and creates anchors. I wanted to add level 6 headers for patterns, but they are almost the same as regular text, which in my opinion looks worse.

4. Fix the funny sentence about 6 types of patterns before a list of 7 items (due to multiple patterns).

5. Add null, false and true to literal table.

6. The shorthands $NodePath and %UniqueNode are not literals.

7. Move warning about comparison since it is not related to the basic syntax

[mhilbrunner]

LGTM, thanks :) Merged.