Add documentation for anchors

[2shea]

Anchors, ^ and $, are used in talon commands to anchor commands at the begging or end, similar to regex anchors (but with some differences).

Examples:

^ means "this can only be matched at the start of a phrase"
$ means "this can only be matched at the end of a phrase"

They can be combined. If you bind ^test one two three$ and you say test one two three four, it should not recognize.

They can be part of a longer sentence, for example, $ so it doesn't try to recognize commands while you're talking.

^wake up$ uses anchors so it can't be recognized mid sentence, which means fewer false positives.

Edge anchoring can change the meaning of a word depending on how it is used. e.g. over$ vs over can be two separate commands.

https://talonvoice.slack.com/archives/C7ENXA7C4/p1621350960040900