Add BaseLabel to the online documentation

[lesamouraipourpre]

• Show adafruit_display_text in the API Reference. This makes BaseLabel visible.
• Change the header in bitmap_label so that it matches the format in label.
• Other minor tweaks.

If we don't want BaseLabel to be visible in the online documentation, just close this PR.

[tannewt]

@jposada202020 Want to review this? I think you added this code.

[FoamyGuy]

I think that I added this code (and did forget to list in in the api.rst file). The PR is referencing the BaseLabel super class that was made during refactoring to eliminate code duplication. @jposada202020 did the work on baseline alignment in both types of labels.

Jose if you have a chance and are up to it you can go ahead and review this.

I will plan on taking a look at it later tonight though if you don't get to it.

[jposada202020]

@FoamyGuy I did take a look of the code this morning, for me the changes are good to go, I have not built the documentation, I was planning to build the readthedocs to be sure that everything was good. I was going to do it also tonight 😄

[jposada202020]

I verified in an online readthedocs of the proposed changes:

• Show adafruit_display_text in the API Reference. This makes BaseLabel visible. - GOOD

• Change the header in bitmap_label so that it matches the format in label. GOOD

• Other minor tweaks. GOOD
  Verifications included returns and return type in __init__py
  Verification that the link in line 168 works.
  

• Tested on

Adafruit CircuitPython 6.2.0-beta.3 on 2021-03-04; Seeeduino Wio Terminal with samd51p19 

Looks good to me

@FoamyGuy I think we could merge after we say thankyou to @lesamouraipourpre 👍🏼

[FoamyGuy]

I built the docs locally and all of these changes are looking great in there.

Thank you @jposada202020 for looking this over, and trying it out. And thank you @lesamouraipourpre for these docs improvements!