Add new boards.fetch_all_items_by_board_id method

[Diosbejgli]

Add new boards.fetch_all_items_by_board_id method to fetch all board items without having to implement pagination by the user as its already done in the API client.

Uses the new next_items_page object in Monday API for cursor-based pagination based on https://developer.monday.com/api-reference/changelog/new-next_items_page-object-for-cursor-based-pagination

Test query:

monday = MondayClient("Secret")

items = monday.boards.fetch_all_items_by_board_id('12345678')

>>> len(items)
1802

Also introduced a new global constant DEFAULT_PAGE_LIMIT_ITEMS with a value of 500 and made it the default for fetch_items_by_board_id and fetch_all_items_by_board_id methods for more efficient queries. Otherwise the API default is 25.

I've also fixed the doc build failures and removed the whole docs/_build/ folder and added it to .gitignore.