Basic concepts with glossary overrides

[rogermparent]

This PR is #1944 with a commit that adds the ability to override a glossary's tooltip entry with a new MD frontmatter field, tooltip.
When tooltip is specified, it will be parsed at build-time and used as the content of the glossary tooltip- otherwise, the previous behavior is used and the MD body forms the tooltip.

content/docs/user-guide/glossary has been changed to show this feature off:

---
name: 'DVC Project'
match:
  [
    'DVC project',
    'DVC projects',
    project,
    projects,
    'DVC repository',
    'DVC repositories',
    repository,
    repositories,
  ]
tooltip: >-
  Initialized by running `dvc init` in the **workspace** (typically a Git
  repository). It will contain the [`.dvc/`
  directory](/doc/user-guide/dvc-files-and-directories), as well as `dvc.yaml`
  and `.dvc` files created with commands such as `dvc add` or `dvc run`.
---

Initialized by running `dvc init` in the **workspace** (typically a Git
repository). It will contain the
[`.dvc/` directory](/doc/user-guide/dvc-files-and-directories), as well as
`dvc.yaml` and `.dvc` files created with commands such as `dvc add` or
`dvc run`.

This page extends for a _while_, elaborating on what a DVC Project is in a way
where it doesn't make sense to put **all** this stuff in a tooltip.

[jeremydesroches]

Thanks @rogermparent ! I'll use it in #1944 and let you know if any issues.

[jorgeorpinel]

Many thanks, @rogermparent ! We'll continue the discussion in #1944