Add General Tools Page & Block Time Estimator (#3052)

[salmad3]

@a-jwc: created a tools page under "general" and a block time estimator component. #2766

Extension of https://github.com/w3f/polkadot-wiki/pull/3052

[salmad3]

tested locally and lgtm. This can arguably exist in the misc. Also, may want to consider enhanced UI instead of using <Form.label> to present o/p

but failing CI as:

Error:  Docusaurus Node/SSR could not render static page with path /docs/tools because of following error:
ReferenceError: Buffer is not defined
    at Object.94636 (main:145176:29)
    at __webpack_require__ (main:174938:42)
    at Object.91051 (main:144917:14)
    at __webpack_require__ (main:174938:42)
    at Object.98287 (main:145246:22)
    at __webpack_require__ (main:174938:42)
    at Object.55159 (main:142269:18)
    at __webpack_require__ (main:174938:42)
    at Module.38136 (main:28293:17)
    at __webpack_require__ (main:174938:42)
[INFO] It looks like you are using code that should run on the client-side only.
To get around it, try using `<BrowserOnly>` (https://docusaurus.io/docs/docusaurus-core/#browseronly) or `ExecutionEnvironment` (https://docusaurus.io/docs/docusaurus-core/#executionenvironment).
It might also require to wrap your client code in `useEffect` hook and/or import a third-party library dynamically (if any).
[success] [webpackbar] Server: Compiled with some errors in 1.54m

@a-jwc

[DrW3RK]

@alfarok closing this PR, as you implemented the core functionality of this one.