[Sidekick Library] Adding blocks

[asthabh23]

Add below blocks to the library:

• [x] Cards
• [x] accordion
• [x] columns - (3-column variant present in fragment is added, the tables' columns layout still needs to be added)
• [x] fragment
• [x] video

[asthabh23]

@volteanu , as discussed please add the columns variant blocks in the doc and update/close this issue once done. Rest of the other blocks I have already covered.

[volteanu]

Added columns - simple variant, and columns (category).