Improve Developer Guide docs

[eh-am]

Few things I noticed while setting up in a new machine:

make build-third-party-dependencies

is such a hindrance (you gotta install a bunch of stuff), and for a lot of people contributing to pyroscope it's not really needed. Maybe we should make the ENABLED_SPIES=none more proeminent.

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error oauth2-mock-server@4.3.0: The engine "node" is incompatible with this module. Expected version "^12.20 || ^14.15 || ^16". Got "17.9.0"

We have to tell people to use nvm (or any other kind of version management like asdf) and follow the node version set up in the .nvmrc file

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Running lerna bootstrap is required, otherwise the webapp fails trying to resolve @pyroscope/flamegraph

21:11:58           frontend | @pyroscope/webapp: ERROR in ./javascript/pages/ContinuousSingleView.tsx 5:0-59
21:11:58           frontend | @pyroscope/webapp: Module not found: Error: Can't resolve '@pyroscope/flamegraph' in '/home/eduardo/work/pyroscope/pyroscope/webapp/javascript/pages'

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Some of the css was broken

Trying again solved it (???)

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It took some time to build the first time
I think it was due to the @pyroscope/flamegraph still being built,
Plus it was difficult to tell when things were ready (ie when should i be able to access localhost:4040 ?)