Update "Running pipx From Source Tree" section

[chrysle]

• I have added an entry to docs/changelog.md

Summary of changes

Running pipx from source isn't possible any more since #1130 (removal of the pipx.version module). Therefore I removed
the corresponding section in CONTRIBUTING.md.

Test plan

Tested by running

# command(s) to exercise these changes

[dukecat0]

Edit: it's because I've run pip install -e . before. Maybe we should replace python src/pipx --version with this command?

Seems like it can still be run correctly:

$ python src/pipx --version
1.4.1.dev5+g98dad34

[chrysle]

Good idea, done. I also removed the enabled argument from the search plugin configuration in mkdocs.yml. It somehow broke the MkDocs build, and is set to true by default anyway.