doc: add tips about lake usage

[joneugster]

Adding some notes about custom setups that are possible with a lakefile.lean, in particular how to use a single shared mathlib in multiple projects.

This might be useful for people with little disk space as well as for teaching setups where it might be reasonable to have a single mathlib on a shared drive and then have students reference this instead of downloading their own copy.

[grunweg]

Thanks for writing this! I have a left minor comments, mostly about copy-editing/grammar.

[joneugster]

Thank you @grunweg for the review! I completely forgot this PR, but now I've addressed all your suggestions

[tydeu]

This PR was mentioned again on Zulip today, so I thought I should take another look. Below are some comments I had on on the shared mathlib section. In regards to the local versions section, the next version of Lake should, hopefully, have some built-in support local versions of dependency, making that section much less hacky. :-)

[tydeu]

Suggested change

	```bash
	rm -rf .lake # because `lake clean` does not remove `.lake/packages/mathlib` which might have been downloaded by `lake new`.
	lake clean # or potentially `lake update -R mathlib` instead
	```
	*(note: it looks like a bug that with a simple `lake clean`, there might still be a folder `.lake/packages/mathlib` floating around from before you changed the `lakefile.lean`. However, deleting `.lake/` is a reasonably safe action as it only contains build artifacts that are fully recovered by the next `lake` call.)*
	```bash
	lake update -R mathlib # use the new path for mathlib
	rm -rf .lake/packages/mathlib # delete the previous local clone of mathlib
	```

lake update -R mathlib should be enough to switch to the relative mathlib. If not, that is a bug. Also, lake clean does not delete Lake packages, it merely removes build outputs.

[tydeu]

Suggested change

	6) When you updated your global mathlib it might be enough to call
	```
	lake update -R mathlib
	```
	which would in theory update everything automatically.
	However, if there are breaking changes to the `lakefile` parsing, you might need to
	* edit `lean-toolchain` to be the same as your global mathlib.
	* make sure `lakefile.lean` parses without error in the new version.
	* try `lake update -R mathlib` again.
	6) Updating your global mathlib should, in theory, update everything automatically.
	However, if there are breaking changes to the `lakefile` parsing, you might need to
	* edit `lean-toolchain` to be the same as your global mathlib.
	* make sure `lakefile.lean` parses without error in the new version.
	* try whatever broke again.

You should not need to lake update a relative dependency. (as path dependencies are not versioned).