Add changelog for 2.0.0b1

[consideRatio]

• for #991

---

Is there something else to include before going for a 2.0.0b1 release?

---

I've recently self-merged these and included them in this changelog.

• 1001

• 1002

---

Link to rendered changelog: https://the-littlest-jupyterhub--1000.org.readthedocs.build/en/1000/reference/changelog.html

I've not done work including v1 -> v2 upgrade instructions. My take is that its relevant to do when there is a distribution specific migration that needs to be done for example, but I think that isn't the case for this major version of tljh, so I've leaned on referencing other software's changelogs.

[minrk]

Thanks @consideRatio !