Update outdated dev notes

[blms]

Updating this list as Mohamed attempts a local build from scratch:

• update python 3.8 --> 3.9 as otherwise may encounter a functools issue
• inquire with RSK about chown step; is this necessary for local install?
• mention that percy and lighthouse tests are meant to be run in CI with github actions and not locally
• mention that running parts of the test suite may result in failures around migrations tests, as they are order-dependent and must be run in the context of the entire test suite
• mention the develop and main branches and what the difference is, which to use based on use case

[mabdellatif88]

• For the first part, I propose changing "3.8" here to 3.9. Don't have the write access to modify the repo
• For the second part, I recommend keeping the Chown command in case of docker / staging / prod server setup and ignoring it in case of local setup (may be simply add a memo that says that and a similar memo for Percy and Lighthouse at the end). Let me check with RSK first

[blms]

Added the two notes from our troubleshooting just now @mabdellatif88!

[rlskoeser]

@mabdellatif88 just sent you an invite to have access to this repository, it would be great if you want to open a pull request with any changes you want to propose based on what you've figured out - clearly we haven't kept these up to date.

Thanks @blms for working with @mabdellatif88 on this.

On the python version, we should probably note that current version is set in .python-version - I think some tools take advantage of that automatically, but forget which (maybe pyenv?). Would be nice not to have to remember to update this version as well.

Confirming that the chown step is specific to docker, since the default user is root but solr user needs to own the configset files.