Update glossary
Update the glossary to the new tokenizer API. I also added a first section about general terms that might confuse a beginner (which anyone is welcome to expand when they see a term that's not necessarily obvious and that we use a lot) and proper references to the subsections about the model inputs to make sure none of the links break.

There was a bad indent introduced since my cleanup of sphinx warnings, so fixed that in passing.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5148?src=pr&el=h1) Report
> Merging [#5148](https://codecov.io/gh/huggingface/transformers/pull/5148?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f45e873910e60d89511ae0193711e71c5c710468&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5148/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5148?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5148   +/-   ##
=======================================
  Coverage   77.19%   77.20%           
=======================================
  Files         133      133           
  Lines       22233    22233           
=======================================
+ Hits        17163    17164    +1     
+ Misses       5070     5069    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5148?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5148/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VsZWN0cmEucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5148/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.28% <0.00%> (+0.12%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5148?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5148?src=pr&el=footer). Last update [f45e873...fde64f8](https://codecov.io/gh/huggingface/transformers/pull/5148?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>That's great! I left some nits.<|||||>Addressed (something wrong with the time in my WSL so the commit appears before the comments....)