docs: Use cosine similarity in quickstart example

[Michael-JB]

• Previously, the quickstart example used dot product distance. This didn't make much sense as the vectors in the example were not normalised. It also led to some weird-looking query results (e.g., searching for the exact vector of the Mumbai point does not return the Mumbai point in the result set). This commit changes the examples to use cosine similarity.
• While I was in the area, I prefixed '/' to the HTTP request paths so that they're all correct and consistent.