Add toc to README and lint contributing.md (#461)

### Description of the change

- Add ToC to README for easier navigation (and add note to `contributing.md`)
- Fix some heading levels in README
- Put upgrading notes into collapsible blocks
- Format `contributing.md` according to MD rules
- Allow `details` and `summary` elements in README
- Allow for longer headings

### Benefits

Better documentation

### Possible drawbacks

None

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/461
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

.markdownlint.yaml

@@ -47,7 +47,7 @@ MD013:
   # Number of characters
   line_length: 200
   # Number of characters for headings
-  heading_line_length: 80
+  heading_line_length: 100
   # Number of characters for code blocks
   code_block_line_length: 80
   # Include code blocks
@@ -106,7 +106,7 @@ MD030:
 # MD033/no-inline-html - Inline HTML
 MD033:
   # Allowed elements
-  allowed_elements: []
+  allowed_elements: [details, summary]
 
 # MD035/hr-style - Horizontal rule style
 MD035: