docs: remove endpoints that wont change and adds the /explorer one

[aleortega]

Reason of removed endpoints:

• POST /content/entities/active
  • Proposed change: receive extended urns for wearables and emotes to return metadata of the associated active entity
  • Reason about push backing the change: the metadata won’t change between individual minted items, meaning that requesting metadata by Asset type is enough. Also, accepting extended urns will impact on how we cache these entities, drastically growing the size of the cache we are using without benefits since the metadata won’t change per minted item but per asset type.
• GET /content/pointer-changes
  • Proposed change: return extended urns for wearables and emotes metadata that have changed during a specific period of time
  • Reason about push backing the change: the metadata won’t change between individual minted items, meaning that requesting metadata by Asset type is enough
• GET /lambdas/collections/contents/{urn}/thumbnail
  • Proposed change: accept extended urns to return thumbnail of the asset
  • Reason about push backing the change: the thumbnail will be the same for each minted item, meaning that the thumbnail is associated to asset type and won’t change per minted item (all of them will have the same). That’s why calling this endpoint without an extended URN is enough
• GET /lambdas/collections/contents/{urn}/image
  • Proposed change: accept extended urns to return thumbnail of the asset
  • Reason about push backing the change: the image will be the same for each minted item, meaning that the image is associated to asset type and won’t change per minted item (all of them will have the same). That’s why calling this endpoint without an extended URN is enough

Besides, this PR also adds the endpoints used by explorer to render the backpack (GET /explorer/:address/wearables)