docs: update docs & benchmarks related docs

[Imod7]

Description

Updated the docs to reflect the recent changes in Sidecar documentation & benchmarks.

This is a result from the changes in Sidecar workflows done by the CI Team. Related PRs:

• ci: Move from Gitlab to Github #1531
• ci: fix benchmark workflow, move docs to gh-pages #1552

Important Note for Contributors & Maintainers

• When you update the documentation in a PR, it is no longer needed to build the docs (with yarn build:docs). You only need to update and commit the src/openapi-v1.yaml file. The docs will be built & published automatically in gh-pages as soon as the PR is merged in master branch.
• Benchmarks are now published in Github pages as well under the URL https://paritytech.github.io/substrate-api-sidecar/dev/bench/. Data will be updated with every commit in master branch. Currently there is only one data point from the last commit.

[IkerAlus]

LGTM