Add a section on dependency management within Splink

[ThomasHepworth]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

Anything marked with [DEPS] and more recently:

• sqlalchemy version conflicts - [FEAT] Remove the sqlalchemy<2 constraint #1906
• Dependency adjustments and the "sunsetting" of python 3.7 for Splink 4 - Update dependencies for splink4 #1933
• Nick's PR to loosen the Splink 3 dependencies - Loosen dependency requirements #1968

Give a brief description for the solution you have provided

This PR aims to add documentation on:

• Our strategy for dealing with package dependencies
• How we deal with end-of-life python versions going forward

This summarises what we talked about previously and adds some additional guidance, where appropraite.

Please let me know if there's anything you disagree with, want added or adjusted.

---

NOTE: It currently points at your edge evaluation guidance because I forgot to checkout onto master and create my new branch. This shouldn't matter. I'll point it towards master once #1927 is merged.

[NickCrews]

This looks like a great thing to think about and write down. didn't read in depth, but I did notice a type of the filename depdenency_management.md

[ThomasHepworth]

This looks like a great thing to think about and write down. didn't read in depth, but I did notice a type of the filename depdenency_management.md

Hahaha, thanks Nick. I'll sort it in the morning.

[RossKen]

Thanks for putting this together! It looks great!  🙌

Just a few minor comments

[RobinL]

Just to say had a skim and this looks good to me too

[RossKen]

This is great! 🙌

Good to go from my end