bury: true support in sitemap

[amitu]

In sitemap, sometimes we want some sections, or some sub-section/toc-item to be "buried" by default. If we have many sections/sub-sections/toc-items at any level, it makes sense to bury them. When something is buried, we will show a single (...) icon for every buried item at that level, and when user clicks on (...) the buried items would be shown.

-- fpm.sitemap:

# Home
# Docs
## Data
## UI
## API
   bury: true

- foo
- bar
- baz
  bury: true
# Learn
# Contribute
  bury: true

In above example we see bury at section, sub-section and toc levels.

[ADITIRAI290698]

Owner: @ADITIRAI290698

Owner: @AviralVerma13

Started On: 7th Nov 2022

Reference Discussion : https://github.com/ftd-lang/fpm/discussions/604
Reference PR : fastn-stack/fastn-old#594

7th Novemeber

• Going through the PR : Icon for sitemap elements fastn-old#594